Prepare the cream: Put sour cream of 20% fat content in a refractory mold, add sugar, egg, vanilla sugar and flour. Mix well, put in a water bath and cook, stirring, until thickened. It took me 15 minutes from the moment of boiling. Cover with cling film to prevent a crust from forming, and allow to cool completely. Then beat the butter at room temperature with a mixer and add it to the cream.