For the dough, beat egg with warm water until thick and creamy, gradually add sugar and vanilla sugar and beat until creamy.
Mix flour with cornstarch and baking powder, sift and gently fold into the egg cream (do not stir). Spread the dough on a 32 x 11 cm baking tray lined with parchment paper and fold the paper into a crease on the open side of the baking tray just in front of the dough to form a border.
Bake immediately at 200-225 °C for 12 min. Immediately after baking, turn the sponge out onto a piece of paper sprinkled with sugar, brush the parchment paper with cold water and peel off carefully but quickly.
For the filling Whip the whipped cream for 1/2 minute. Sift confectioners’ sugar, combine with cream stiffener, and beat until incorporated throughout. Whip the whipped cream until stiff.
Cut the cooled sponge cake into 2 11 x 16 cm strips and spread one strip with just under a third of the whipped cream. Place the ice cream on top and also spread with just under a third of the whipped cream. Place the other pastry strip on top. Spread the entire pastry with the remaining whipped cream, sprinkle the top with grated chocolate and place the pastry in the freezer of the refrigerator for a few hours.