Mix egg yolks with sugar. Sift 2 tablespoons flour into egg yolk mixture and mix well. Pour in the cold milk and mix again.
In a saucepan slowly heat 125ml milk (do not bring to the boil). Very gradually start adding egg yolk mixture, whisking constantly. Cook on low heat until mixture thickens slightly. Take off heat and cool to room temperature.
Mix softened butter with a fork and gradually add the cooled egg mixture, starting with 1 to 2 tablespoons at a time. Then increase to 3 to 4 tablespons at a time and keep beating with fork. When an even, fluffy consistency is reached, beat with hand mixer to incorporate more air and add volume.
Add 1 to 2 teaspoons vanilla extract or more sugar at the very end, for added flavour.