For the Barbara cake, cream the kitchen-warm butter until fluffy. Slowly add the powdered sugar, vanilla sugar, lemon zest and eggs.
Continue mixing until the powdered sugar has dissolved. Sift together the cornstarch, flour and baking powder and add to the batter, a tablespoon at a time.
Bake the mixture in a buttered floured pan at 160 degrees for about 80 minutes. For the glaze, mix the sifted powdered sugar with lemon juice and pour on the cake while it is still hot.
Let the Barbara cake cool before serving.