Cream the butter, then add the yolks, beating and mixing thoroughly. Mix in the sour milk or cream and the sugar. Finally add the whites of eggs beaten to a stiff froth. Add sufficient flour for the mixture to be firm enough to roll into small balls. Poach them in boiling water for 5 minutes. Have ready a frying pan with melted butter and breadcrumbs, put the quenelles in this, shake the pan, so that they will be evenly coated and slightly browned. Remove from the pan and sprinkle with sugar. The sugar in the mixture is often omitted and the quenelles are merely sprinkled with sugar when they are done.