Beat the eggs with the sugar and vanilla in a mixer until it turns pale and creamy. This takes a while and requires high speed. While beating the egg mixture, preheat the oven to 350°F (180°C). Line a 20 cm (8-inch) springform pan with parchment paper and butter and flour the sides. Shake off the excess flour.

Mix the flour, baking powder, salt, and cocoa powder in a bowl and sift them twice.

Gently fold the flour mixture into the egg mixture, one spoonful at a time, using slow circular motions from bottom to top. Bake for 25 to 30 minutes.

Test if it's fully baked with a skewer; if it comes out clean, it's done. Also, when properly baked, it will pull away from the sides of the pan. Remove from the oven and let it cool completely. Once cooled, remove from the pan, wrap in plastic wrap and aluminum foil, and refrigerate until the next day.

Prepare the marbling for the next day. Melt the two types of chocolate broken into pieces in two separate bowls. Melt them in a double boiler or microwave, but do not let them fully melt in the microwave. Keep the chocolate temperature low to maintain its shine when it cools again.

Place a piece of parchment paper on a flat surface; I placed it on an upside-down baking sheet. Pour the dark chocolate first to form a rectangle. Drag a toothpick vertically through the white chocolate lines from right to left, then move the toothpick slightly lower and drag it vertically through the white chocolate again from right to left, continuing downward.

Let it cool and set, then cut it into any shape you like with a straight knife (not serrated). Use any leftover or broken pieces for decorating the cake by sprinkling them around the marbling like small soil pieces.

Make the pastry cream. Heat the milk and mix the egg, sugar, and cornstarch in a bowl. Just before the milk boils, add it to the pot and stir continuously. Add the butter, and just before it thickens, add the chocolate pieces and vanilla. Once thickened, let it cool covered with plastic wrap touching the surface of the cream.

Sift in the powdered sugar, then close the mixer with its lid and cover with a towel to prevent the sugar from escaping. Beat on medium speed until it becomes a thick cream.

Stop the mixer, sift in the cocoa powder, add the melted chocolate, and beat again until the chocolate is incorporated into the cream. Finally, add the salt and mix a little more.

When assembling, everything should be cooled. Place the springform ring without the base on your cake stand. Beat the pastry cream in the mixer to fluff it up and put the buttercream in a piping bag.

Cut the piping bag at the tip and apply buttercream all over the cake. Reserve some cream in case you need to fix anything when smoothing or for decoration on top of the cake. Smooth the cake all around and on top with a bread knife. Refrigerate the cake again.

Decorate the cake with the marbling and any leftover buttercream.