Chocolate cake: dilute cocoa powder with hot water. Set aside.

Whisk egg yolks along with 12 grams of sugar. Add oil and mix until well-combined. Then add milk and mix well.

Sieve in the flour in two batches. Mix after each addition until the batter is smooth. Mix in the diluted cocoa. Set aside.

Beat egg white until frothy with an electric mixer running at high speed. Then gradually add sugar as you mix. Continue beating until it almost reaches stiff peak stage.

Mix in 1/4 of beaten egg white into the cake batter using a spatula or a hand whisk. Then add another 1/4 of egg white. From here on, we want to gently combine the mixture. Be careful not to deflate the batter. Again, mix in another 1/4 portion. Add the remaining egg white. Mix just until no white streaks are seen.

Pour batter into an 11-inches (28 cm) square pan that has been lined with parchment paper. Spread the batter evenly. Tap the pan twice onto the counter to remove air bubbles.

Bake in a preheated oven at 285°F (140°C) for 20 minutes, then increase to 320°F (160°C) and continue baking for 15 minutes. Once baked, remove the cake from its pan and let cool completely.

Buttercream: beat the softened butter with an electric mixer until light and fluffy. Add condensed milk and mix just until combined.

Cut and divide the cake equally into two. Spread the buttercream over one piece of the cake. Cover it with the other piece to make a sandwich.

Divide the cake equally into three rectangular pieces. Then cut each piece diagonally from one corner to the opposite corner to get triangular cake slices.