Heat up 2/3 of milk quantity with sugar till sugar melts fully. Put together an egg (55gr.), flour and 1/3 quantity of milk and stir until smooth. Very accurately pour the egg mix into slightly heated milk with sugar, stir intensively. Cook till the mixture thickens (up to 85⁰C), do not stop stirring. Tightly cover the mixture with plastic film and cool to reach room temperature, whip butter until white , continue whipping and add the mixture in three portions.