Melt the Butter in a saucepan over a medium heat.
Add the Plain flour and stir continuously to form a smooth roux.
Cook the roux for 2 minutes, stirring constantly.
Gradually add the Whole milk, whisking continuously after each addition to create a smooth sauce.
Continue cooking for 5–10 minutes, stirring frequently, until the sauce thickens to the desired consistency.
Season with Sea salt and White pepper to taste.
If a smoother texture is required, blend briefly with a hand blender.