Bake brownies as described on Culinary in the Desert. For the topping, combine 200 g of milk and 3 g of agar (= agar for about 300 ml of liquid). Mix 25 g of water and 25 g of vanilla sugar and bring to a boil. Pour in the milk, bring to a boil again and let simmer for 2 minutes. Take from heat, let cool a few minutes. Stir in 250 g of sour cream. Pour over the cake immediately. Let solidify. Refrigerate, if wanted.