Melt the butter in a pan over a low heat and mix in the flour until it forms a paste (also known as a roux).
Cook the roux over a low to medium heat for a few minutes, stirring as you go, until it leaves the sides of the pan easily.
Slowly add the milk to the roux bit by bit, whisking as you go. When all the milk has been added simmer gently for up to 20 mins stirring regularly until you can no longer taste any flour.
When your sauce forms a smooth, shiny and thick consistency it’s ready to be seasoned and either flavoured or cooled for storing.