Avocado Chiffon Cake:

Separate 6 eggs into yolks and whites, and let them come to room temperature.

Preheat your oven to 340 F (170 C). Prepare two 9" round cake pans by lining the bottoms with parchment paper. Do not grease the parchment paper or the sides of the pans. Soak 2 cake strips in cold water for 5 minutes. Then shake off excess water but do not wring. And place the soaked cake strips around the cake pans.

In a food processor, puree 1 ripe avocado. Set this pureed avocado aside.

In a large mixing bowl, sift together 1 2/3 cups of cake flour, 2 tsp of baking powder, 1/3 cup of granulated sugar, and 3/4 tsp of salt. Whisk together to combine. Then add in the pureed
avocado, 6 egg yolks, 1/3 cup of vegetable oil, 1/2 cup of water, and 2 tsp of vanilla extract.

Beat this together with an electric mixer until the mixture is smooth and well blended.

In the bowl of your stand mixer, add in the 6 room temperature egg whites and 1 tsp of cream of tartar, and with the whisk attachment beat on medium speed until it looks frothy, about 2 minutes. Then turn the speed up to medium-high, and very slowly add in 1/2 cup of granulated sugar while continuing to beat. When all the sugar has been added, turn the speed up to high and beat until you see stiff glossy peaks.

One third at a time, gradually and very gently fold in this meringue into the cake mixture until very well combined. Distribute the batter into the prepared round cake pans, with approximately 1/3 of the batter going into 1 pan and approximately 2/3 of the batter in the other pan.

Bake in the preheated oven at 340 F (170 C) for about 30 to 35 minutes, or until a toothpick inserted into the center of the cake comes out clean and the top surface springs back when gently pressed. In our experience, the pan with 1/3 of the batter will take about 30 minutes, while the pan with 2/3 of the batter will take about 35 minutes.

Use a sharp knife to go around unsticking the cake from the sides of the pans, then immediately invert the pans onto metal racks and lift off the pans. Let the cakes cool on the metal racks to room temperature.

Once the cakes have reached room temperature, the cake with 2/3 of the batter horizontally in half. Set aside the 3 layers of chiffon cake.

Avocado Italian Buttercream Frosting:

Separate 3 eggs into yolks and whites while they are still cold - save the yolks to use for another dish. Let the 3 egg whites come to room temperature. Take 1 1/2 cups (3 sticks) of butter and slice them into 1-inch cubes, and let them come to room temperature.

In the bowl of your stand mixer, add in the 3 room temperature egg whites and 1/4 tsp of cream of tartar. With the whisk attachment, beat this on medium speed. Slowly add in 1/2 cup of granulated sugar, and continue beating till you see soft peaks forming. Let this rest.

Take a small saucepan and add in 3/4 cup of granulated sugar and 1/4 cup of water. Heat it over medium-low heat. Wait for the sugar to dissolve completely and the mixture looks clear.

When the sugar has dissolved, attach a candy thermometer and turn up the flame to the highest level possible. Without stirring, continue heating till the sugar-water mixture reaches the softball stage, which is 235 F (110 C). Pay very close attention to the thermometer. If the sugar gets heated passed 235 F, your resulting frosting will be grainy.

As soon as you see the thermometer read 235 F (110 C), remove the saucepan from the heat. Turn on the mixer on medium-low speed, and immediately drizzle the hot sugar mixture into the egg-white meringue in a slow steady stream. The sugar should be hot enough to be in the soft-ball stage so it stabilizes the eggs, as well as hot enough to pasteurize the egg whites so they are safe to eat.

When all the hot sugar syrup has been added, continue running the mixer until the mixture cools completely to room temperature, which could take 15 to 20 minutes or so. Do not worry, as you cannot "overmix" this meringue. Letting the mixer run just helps to cool down the meringue faster. It should end up having a nice silky-smooth texture, and it should be the same temperature as the room temperature butter before they are combined.

When the meringue has cooled completely to room temperature (after 15 to 20 minutes of mixing on medium-high), turn the speed down to medium-low. One cube at a time, add in the 1 1/2 cups of room temperature butter, waiting for each piece of butter to get incorporated before adding the next one.

If the mixture starts looking curdled, do not worry. It just means the mixture is too cold and it will become smooth once it warms during mixing. Again, buttercream cannot be "over-mixed". If the mixture starts looking soupy, do not panic either. It just means the mixture is too warm (tends to happen on a hot day). Just put your bowl in the fridge for 10 - 20 minutes for the mixture to firm up.

Once all the butter has been incorporated, add in 2 tsp of vanilla extract and 1 tsp of salt. Turn the speed back up to medium-high, and mix them until smooth.

In a food processor, puree 2 ripe avocados. Add this avocado
puree into the buttercream frosting. Continue mixing the buttercream on medium-high speed for another 5 minutes, or until it's well-incorporated, smooth, and has a silky consistency. Do not worry about over-mixing, as you can't really overmix meringue-based buttercreams.

For a more vibrant light green color in your frosting, you can add 4 drops of green gel coloring at this point, but this is optional. We use a gel paste food coloring called "leaf green". To do this, take 1 tbsp of frosting and transfer it to a separate microwavable bowl. Add 4 drops of gel food coloring to this 1 tbsp of buttercream, and microwave it for 10 seconds. Microwaving this small portion will allow the colors from the food coloring to take to the buttercream, as food coloring does not readily dissolve into buttercream.

Once you have this small portion of buttercream with activated coloring, mix it back into the remaining Italian buttercream frosting. Add more or less coloring, 1 drop at a time using that microwaving technique, to achieve the color you want. We're only using 4 drops here for this, for more natural pale green color.

Assembling the Cake:

Place a cake board that's larger than 9" (we use a 10" cake board) on a revolving cake stand. Place one of the chiffon cake rounds on the cake board.

Spread a thin layer of the avocado buttercream frosting mixture on top, creating an even and flat layer as much as possible, about 1/4" thick layer of frosting.

Place the second layer of avocado chiffon on top. Again, spread avocado frosting on top to cover the second layer, about 1/4" thick.

Place the third and final layer of cake on top. Spread avocado frosting to cover the top and sides of the cake.

If you have an icing comb, you can create a pattern on the outside surface of the frosting at this point. I'm just using the icing comb to create a pattern around the sides of the cake. Then just clean up and smoothen the top edges and surface of the cake as much as possible. Clean up excess frosting at the base as needed.

Fit a piping bag with an open star tip (I like to use a #21 tip for this), and transfer the remaining avocado buttercream into the piping bag. Create a shell border around the bottom of the cake. Then do the same thing around the top surface, right at the edge of the cake. To make the shells as evenly sized and shaped as possible, try not to lift your tip when pressing. Just press to create the shell bulb, then decrease pressure while dragging to create the tail.

Spread about 1/2 cup of chopped pistachios evenly on the top surface, making sure they reach the frosting wall.

Refrigerate overnight for the layers to set, and set it out at room temperature for about 20 minutes before serving to let the Italian buttercream soften a bit. Enjoy!