Meringue:

Separate 10 eggs into yolks/whites, and let it come to room temperature. Remove 2 cups of butter (4 sticks) from the fridge and also let it come to room temperature.

Preheat oven to 350 F (175 C). Line baking sheet with parchment paper.

Spread all cashews (3 cups total) in an even layer in prepared baking sheet. Bake for 10 minutes or until cashews are dark golden brown. Set aside and cool completely.

Place 2.5 cups of cooled toasted cashews and 1 cup of confectioners’ sugar in the bowl of a food processor fitted with blade attachment. Pulse mixture in quick short bursts. Scrape down sides of bowl as needed to ensure even grinding. Continue to pulse until mixture resembles even, coarse sand. Transfer to a bowl, and set aside.

NOTE: stop pulsing in food processor when you see "coarse grains" of cashews, but should not be a powder.

Reduce oven temperature to 300 F (150 C). Trace four 8 x 8-inch squares on parchment paper. Flip paper over so ink is on underside. Set aside.

NOTE: If using half size (13x18-inch) baking sheet trays, you can fit two 8 x 8-inch squares per tray.

Place 10 room temperature egg whites in a clean mixing bowl. Add in 1 tsp of cream of tartar (optional - but helps stabilize egg whites). Use a stand mixer fitted with whisk attachment or electric hand mixer to beat egg whites and cream of tartar at low speed until foamy. Once foamy, slowly add 1 cup granulated sugar one tablespoon at a time while beating at medium speed. Continue to beat at medium speed until all the sugar has been added. Then, increase to high speed and beat egg white mixture till you see shiny, stiff peaks.

Use a large spatula to gently carefully fold ground cashew mixture into egg whites one-third at a time.

NOTE: Do not over-mix. Over-mixing will cause whipped egg whites to lose volume.

Transfer batter to a piping bag fitted with the biggest round piping tip you have. Or with no tip at all (just hole at tip of bag). Follow the traced squares on prepared parchment lined baking sheets to pipe four 8 x 8-inch squares. Start at the outer edge, of each of the 4 squares. Then make a smaller cake within each of the 4 squares, and keep doing till you fill all 4 squares. If you run out of meringue before you finish filling in the middle of the square, that's okay. Just spread the meringue you have into the middle, so that the square is as flat and even as possible.

Bake in preheated 300 F (150 C) oven for 30 to 40 minutes (depending on oven) until meringue is dry. As soon as meringue squares are taken out of the oven, invert them onto wire racks. And let them cool to room temperature.

NOTE 1: It is very important to remove meringue squares from parchment paper and invert them onto wire racks immediately, while they are still hot. Otherwise, meringue will stick to parchment paper on baking sheet it was baked in.

NOTE 2: For dry and crispy cashew meringue wafers (rather than the soft and moist wafers in the video), bake the meringue wafers at 225 F (105 C) for 5 to 6 hours instead. The lower temperature over a long baking time will remove all the moisture from the egg whites creating a hollow and crispy texture, without burning the outer layers of the meringue.

French Buttercream:

Place 10 egg yolks into a heatproof bowl (either glass or stainless steel). With an electric mixer, whip on medium speed for about 5 minutes, until thick and lemon-yellow in color.

Meanwhile, in a small heavy bottomed sauce pan combine 1 cup of sugar and 6 tbsp of water. Set over low heat, and stir a little till sugar dissolves. Once sugar has dissolved, place a candy thermometer into pot. DO NOT STIR from now on. Turn the heat to highest possible, and heat mixture to soft ball stage, till thermometer reads 235 F (110 C). Immediately remove from the heat.

With the mixer running on slowest speed, carefully drizzle hot sugar syrup into whipping egg mixture in a slow steady stream. This syrup needs to be hot enough so it's at the soft ball stage, and to also "cook" and pasteurize the egg yolks to make them safe to eat. Once all the sugar syrup has been added, continue to whipping on low speed until mixture has cooled to room temperature, or about 75 F (25 C), which should take about 15 to 20 minutes.

When yolk-sugar mixture has cooled to room temperature, with the mixer running on low speed, add the 2 cups softened room temperature butter one cube at a time, allowing each piece of butter to be incorporated before adding the next. Once all the butter has been added, mix in 2 tsp vanilla, 1/4 tsp of salt, and an optional 2 tbsp of dark rum. Continue to beat until buttercream is silky and smooth - another 2 minutes.

NOTE: The butter will melt if the egg-sugar mixture is too hot. If you notice the butter separating, cool down the sides of the mixing bowl with by placing in an ice-bath Alternatively, you can put the bowl in the freezer for a few minutes to cool contents down before adding in more butter. Eventually, the buttercream should come together.

Assembly:

Take remaining 1/2 cup of toasted cashews and pulse them in clean food processor till you see rough chopped grains of cashews (but not too much to become powder).

Dab some icing in the middle of serving plate to prevent cake from sliding around. Place one cashew meringue square on cake board or serving plate. Spread about 3/4 cup of buttercream in an even layer over the meringue square. Sprinkle about 2 tbsp of the roughly chopped cashew grains over the layer. Repeat stacking cashew meringue squares, 3/4 cup buttercream frosting, and 2 tbsp cashew grains. With the 4th meringue square on top.

Use remaining buttercream to coat sides and top of cake.

Decorate the top and sides of the cake with tines of fork in whatever pattern you like. I like to make a crisscross pattern on top, and then alternating up-down and sideways pattern on the side of the cake.

Finally, sprinkle the top of cake with the remaining chopped toasted cashews. Refrigerate for a minimum of 2 hours, ideally overnight, for the cake to set completely. Keep it chilled until ready to serve.

Let cake sit at room temperature for at least 30 minutes to let the French buttercream soften before slicing and serving. Enjoy!