Place an oven rack in the middle position, and preheat your oven to 350 F (175 C).

Take a 9" x 5" loaf pan, and butter the sides and the bottom of the pan. Place a piece of parchment paper along the long side of the pan such that the parchment paper goes up the sides and can be used as a handle to lift out the banana cake after baking. Butter the parchment paper and flour the sides and bottom of the pan. Let 2 eggs come to room temperature.

In a small mixing bowl or measuring cup, mash 3 large very-ripe bananas so that it makes about 2 cups of mashed bananas. In a separate large mixing bowl and a sieve, add 2 cups of all-purpose flour, 1 tsp of baking soda, 1/2 tsp of baking powder, 1 tsp of salt, and 1 tsp of cinnamon. Sift these together and whisk to combine.

In the bowl of your stand mixer fitted with a paddle attachment, add in the 2 cups of mashed bananas, 1/2 cup of vegetable oil, 1 packed cup of dark brown sugar, the 2 large room temperature eggs, and 2 tsp of vanilla extract. Mix on medium speed till these wet ingredients are well combined. Stop the mixer and scrape down the bowl and paddle attachment.

With the mixer on low speed, slowly add in the flour mixture. When all the flour mixture has been added, continue mixing just until combined. Avoid over-mixing the batter. Scrape down the bowl and use your spatula to mix in any pockets of flour that remain. Then add in 1/2 cup of chopped walnuts, and gently fold it in.

In the prepared loaf pan, add in a thin layer of batter to cover the bottom surface of the pan and spread it out evenly. Spoon about 1 1/2 cups of ube halaya, which is an entire 22 oz jar of ube halaya, like a log along the middle of the bottom surface on top of the layer of banana cake batter. Don't let the ube halaya touch the ends of the pan so that the ends of the ube filling will be covered with batter. Then pour the remaining banana cake batter over the ube halaya and smooth out the surface as much as possible. In a small bowl, add in 1 tbsp of granulated sugar and 1/2 tsp cinnamon. Mix these together, and sprinkle it over the batter. Before placing the loaf pan in the oven, mould a piece of aluminum foil on top so that the foil is the shape of the top of the pan. Remove the piece of aluminum foil and save it for use later.

Place loaf uncovered in the oven and bake on the center rack at 350 F (175 C) for 40 minutes. Then carefully place the prepared piece of aluminum foil loosely on top and bake for another 15 to 20 minutes, or until the top surface at the center looks set, and sharp knife inserted in the center comes out clean. If you have a digital thermometer, the center of the cake should read about 205 F (95 C) when it is done.

Let the finished banana cake cool in the pan for 10 minutes. After 10 minutes, use a sharp knife to unstick the sides and corners without parchment paper as needed. Then use the parchment paper handles to lift the cake out of the pan and onto a cooling rack. Let it continue cooling till desired temperature. Enjoy warm or at room temperature!