Preheat the oven to 175°C (347°F), conventional heat (no fan), with both top and bottom heating elements on.

Add the vinegar to the milk, stir, and set aside.

Heat the butter, oil, and chocolate over low heat until melted, then set aside to cool slightly.

Whisk the eggs and sugar until pale, fluffy, and doubled in volume.

Add the milk mixture and the cooled melted chocolate mixture, then mix for another minute.

Sift in the flour, cocoa powder, baking powder, and baking soda, and gently mix into the wet ingredients.

Pour the batter into a cake pan lined with baking paper and bake for 30 minutes.

Bake 3 cake layers.

Pour the whipping cream into a saucepan and add the finely chopped chocolate.

Heat gently while stirring constantly, making sure the mixture does not boil, until the chocolate has completely melted.

Once cooled, first whip the mascarpone for 2 minutes.

Then add the cold chocolate mixture and whip until light and airy (about 3–4 minutes on high speed).

Finally, take a cup of the light mousse cream and fold it into this dark mousse cream.

Place the chocolate and butter over a double boiler, stirring constantly to prevent burning.

Lightly beat the egg yolks with a fork.

When the chocolate and butter have melted, remove from the heat and immediately pour in the egg yolks, stirring vigorously.

Return the mixture briefly to the double boiler, whisking continuously until everything combines into a smooth, glossy, non-sticky mixture resembling pudding.

Remove from the heat and cover directly on the surface with plastic wrap. Allow it to cool completely.

Whip the cooled chocolate base together with the mascarpone.

In a separate bowl, whip the cream until stiff peaks form.

Fold it into the chocolate mixture and mix briefly until fully combined.

Place everything in a saucepan and stir over heat until it comes to a boil.

Allow it to simmer for 3–5 minutes, until slightly reduced and lightly syrupy.

Adjust the simmering time according to the width of your saucepan, the wider the pan, the faster the liquid evaporates and reduces.

For the neatest layers, assemble the cake inside a cake ring and/or with acetate cake collar.

Soak each sponge layer with several tablespoons of syrup.

Divide both creams into two equal portions.

Layer as follows: Sponge layer, Syrup, Half of the dark cream, Half of the light cream.

Repeat the process once more, then finish with the final sponge layer on top.

Decorate the outside of the cake as desired (whipped cream, buttercream, ganache, fondant, etc.).

Refrigerate overnight before serving.