In a bowl cream butter and cream cheese until light and fluffy. Gradually add flour and stir well. Cover and place in the fridge for 1 hour. Remove cooled dough from the fridge and form 3cm balls and place them into greased mini-muffin tin holes. Press them into the base and sides of the muffin tin holes, to form a case for each brownie mixture.
Preheat the oven to 160 C / Gas 2/3.
In a small saucepan, or double boiler, melt chocolate and butter over low heat. Remove from the heat, allow to cool and stir in sugar, vanilla sugar and egg. Fold in walnuts and evenly spoon mixture into the prepared cases. Sprinkle with coloured sprinkles, if you like.
Bake in the preheated oven for 30 to 35 minutes, until a skewer inserted in the middle comes out clean.
Allow to cool for 10 minutes in the baking tin, then take out and let cool completely on a cooling rack.
Vanilla sugar is generally available in supermarkets nowadays, however, in a pinch substitute 1 teaspoon of vanilla extract to every 2 teaspoons vanilla sugar.