Separate yolks and whites. Beat the yolks to a creamy white color. Start whipping whites, add sugar, whisk until soft peaks. Add whipped egg yolks, beat until completely combined. Sift half the flour, corn starch, cocoa powder and vanilla sugar, mix with a spatula. Sift the remaining flour and this time mix with a whisk.
Grease the mold (approximately 28×20 cm.). Pour the dough. Bake for 25-30 min at 175C/340F. Let it cool.
In a saucepan, combine water, sugar, and vanilla extract. Bring to a boil over medium heat and boil for 3-4 minutes.
Add the powdered sugar and vanilla sugar to the cream, beat until slightly thick, add the mascarpone and beat until a smooth cream is dense.
Turn the biscuit over, cut off the top, cut into two parts.
Soak the first part of the biscuit with syrup. Put the cream, cover with the second part of the biscuit and press lightly to spread the cream. Remove excess and fill voids. Soak the second biscuit with syrup. Put in the refrigerator for 2 hours.
Prepare the icing. Take hazelnut cream, add dark chocolate, melt in a water bath or in the microwave. Mix until smooth, add vegetable oil and mix again.
Cover the baking sheet with parchment, put 2-4 cups, place the cake on them, pour the icing, distribute it so that the sides are also covered, while the excess drains. Leave for a while, so that the glaze grabs a little. With the remaining glaze make a decorative pattern in the form of intersecting waves.
