Mix together yolks, sugar and flour

Add milk and keep mixing until you get a liquid cream without clots. In case there are, use a strainer to melt them

Add the flavor and mix

Cook at low flame mixing continuously to avoid clots. In case some of them start creating, take the cream away from the heat, mix faster to melt them and then put it back on the flame. Turn off the cooker as the cream gets dense enough

Cool it and keep it in the refrigerator before serving