Preheat the oven to 180 degrees Celsius (350 degrees Fahrenheit).

In a saucepan, combine milk, baking powder, condensed milk, cornstarch, and cream.

Bring the mixture to a boil over medium heat, stirring frequently.

Continue to cook, stirring constantly, until the mixture thickens.

Remove the thickened mixture from the heat and allow it to cool completely.

In a separate bowl, beat egg yolks with margarine and brown sugar until a thick and creamy mixture forms.

Gradually add cocoa powder, melted dark chocolate, flour, baking powder, and milk to the egg yolk mixture.

Beat until all ingredients are well combined.

In another clean bowl, whisk egg whites until stiff peaks form.

Gently fold the whisked egg whites into the cake batter using a spatula.

Grease a cake tin.

Pour approximately half of the cake batter into the prepared cake tin.

Carefully pour the cooled filling over the center of the batter, avoiding the edges.

Pour the remaining cake batter over the filling to cover it completely.

Bake in the preheated oven for approximately 35 minutes, or until a toothpick inserted into the center comes out clean.

Remove the cake from the oven and let it cool completely.

While the cake is cooling, add a touch of cocoa powder to the leftover filling mix and stir lightly.

Frost the cooled cake with the cocoa-infused filling.

Decorate the cake as desired.