Preheat Your Oven: Start by preheating your oven to 180 degrees Celsius (or 160 degrees for circulating air). Prepare a springform pan (Ø 26 cm) by lining the bottom with baking paper.
Prepare the Biscuit Mixture: In a large bowl, whisk together 6 eggs, 600 g of sugar, vanilla sugar, and a pinch of salt using a hand mixer. Beat until foamy and light—this should take about 5 minutes.
Fold in Dry Ingredients: In another bowl, mix the flour and cornstarch. Gradually lift this mixture under the egg-sugar concoction until well combined. Pour the dough into the prepared springform pan.
Bake the Cake: Place the pan in the oven on the lower rack and bake for about 45 minutes. Once done, release the biscuit from the mold and let it cool completely on a baking rack.
Make the Cream: In a small bowl, beat 2 eggs with about 2 tablespoons of sugar. In a saucepan, combine the milk and remaining sugar and bring it to a boil. Slowly add the sugar-egg mixture to the pan while stirring continuously. Simmer for about a minute, then pour into a bowl to cool. Once cooled, mix in the chopped dark chocolate and butter.
Assemble Your Rum Bomb: Cut the cooled biscuit in half horizontally to create two layers. Place one layer back on the plate, spread the creamy mixture on top, and then gently place the other half on top. If you have any leftover biscuit pieces, feel free to crumble them on top for decoration!