Mix the butter, sugar, vanilla sugar, orange peel and Grand Marnier until frothy. Sift the flour, cornstarch and baking powder. Eggs and flour are stirred alternately with speed 1 below. Line the loaf pan (25/10/7 cm) with sandwich paper. Pour in the mixture and bake at approx. 160 ° C for 45 minutes. If the cake has a small skin (after approx. 10 minutes), carefully cut into it with a sharp knife so that the cake can rise evenly.