Start by removing the seeds from the vanilla pod. Grate the lemon peel from a lemon and be careful not to include the white part. It is bitter. Squeeze the juice from half of the lemon. Whisk egg yolks, sugar, lemon peel, and vanilla seeds together until it forms a thick, creamy mixture. It is easiest to do this in a stand mixer. Add the ymer while stirring, then the buttermilk, and finally the lemon juice. The cold buttermilk soup is ready to serve. Serve with Danish biscuits (kammerjunkere).