In a bowl, place half of the milk, half of the sweetener, and a pinch of salt. Mix. In a separate bowl, place the flour, baking powder, and a pinch of soda. Mix. Combine the contents of the two bowls. Mix and rub until you get a uniform, thick mass (without lumps).

Pour half of the mass into a small heat-resistant dish.

To the rest of the mass, add cocoa (leave a bit of cocoa for making the topping at the very end). Mix and rub thoroughly until combined.

Add the dark mass to the light one. Gently stir to create a marbled pattern. Bake in an oven preheated to 180°C (356°F) until risen and golden, about 25-35 minutes. Meanwhile, prepare the custard.

In a glass, place light custard powder (e.g., vanilla, cream, banana, or other) and the rest of the sweetener. Pour a small amount of water (about 30-50 g) and mix thoroughly until you achieve a uniform consistency (without lumps).

In a saucepan, place the rest of the milk. Put it on medium heat. When it starts to gently boil – reduce the heat and start pouring in the mixed custard powder in a thin stream (stir again just before pouring to make sure nothing is left at the bottom). Constantly stir at medium speed with a kitchen whisk or spoon. Once all the ingredients are combined, keep stirring and increase the heat to medium. Stir the custard until it boils and thickens. After a moment, turn off the heat. The custard should not have any lumps. If lumps appear, try to get rid of them by more thorough mixing and mashing. Set aside.

Let the baked base cool slightly. Dissolve cocoa in a small amount of boiling water.

Spread custard on the sponge base. Pour over with cocoa topping. Store the dessert in the refrigerator and consume within 2-3 days.