Preheat your oven to 180°C (350°F). Grease and flour two round cake pans.

Separate whites from yolks. Beat the whites with a pinch of salt until foamy, then slowly add 50g of sugar while beating until stiff, glossy peaks form.

Mix the yolks with the remaining 50g of sugar until pale and creamy. Add the room-temperature water and combine.

Sift together the flour, cocoa powder, and baking powder. Add this to the yolk mixture and mix until smooth.

Carefully fold the whipped egg whites into the batter to keep the sponge light.

Divide the batter evenly into the two pans, smooth the tops, and bake for 40 minutes, or until a toothpick inserted in the center comes out clean. Cool in the pans for 10 minutes, then transfer to wire racks.

In a saucepan, whisk together the eggs, sugar, starch, condensed milk, and regular milk. Cook over low heat, stirring constantly, until thickened. Allow to cool completely.

Beat the softened butter with vanilla, then gradually add the cooled cream mixture until smooth and fluffy.

Once the cakes are cool, slice each one horizontally to create four layers. You may soak the layers lightly with sweetened milk if desired.

Spread the creamy filling between each layer, then stack them neatly. Chill the cake for 3–4 hours to set.

Heat cocoa, sugar, and sour cream over low heat until thick and glossy. Spread over the chilled cake to decorate.