Pre-heat the oven to 350ºF (175ºF).
Beat the butter, peanut butter and sugar together until nice and light. Beat in the egg.
Add the flour, baking soda and vanilla sugar. Pour into a greased 6 inch springform pan and bake for around 14 minutes.
Using the back of a spoon, smooth down the top (it should have risen a little), so that you have a fairly flat surface. Allow to cool completely.