Melt the chocolate and butter together in a double boiler or microwave-safe bowl.

Pour the melted chocolate into a mixing bowl and set aside.

In the same bowl (no need to clean), beat the egg yolks and sugar together until pale and fluffy. Add the melted butter, melted chocolate, milk, flour, baking powder, and fleur de sel. Mix until the batter is light and airy. Transfer to a separate bowl.

In a clean, dry bowl, beat the egg whites with a pinch of salt until stiff peaks form.

Gently fold the beaten egg whites into the chocolate batter, being careful not to deflate them.

Pour the batter into a cake pan of your choice. Bake in a preheated oven at 350°F (180°C) for 10 minutes, then lower the temperature to 300°F (150°C) and bake for another 35 minutes. Let cool on a wire rack.

Slice the cake in half horizontally. Spread the whipped cream inside, then replace the top layer. Decorate with chocolate or your favorite treats.

Chill until ready to serve.