Preheat oven to 180 C / Gas 4 and line a baking tray with baking parchment.
Rub together the butter and plain flour in a large mixing bowl then mix in the caster sugar.
Add the vanilla and make sure you mix it very well throughout the dough. Then beat in the egg yolk lightly. Do NOT throw away the egg white. The mixture will still be a little crumbly.
Mix in the milk and if needed add one more tablespoon. The mixture does not need to be perfect, it just needs to be smooth. If you add too much milk, add more flour.
Tip your dough onto a clean board or countertop and roll it out to the thickness of two one-pound coins. Cut the dough with cookie cutters.
Pop the shaped dough onto your baking parchment, and before you put them in the oven make sure to brush them with your leftover egg white from earlier. This will give them that beautiful golden baked colour.
Place the cookies in the oven for 15 minutes and test them by putting a wooden skewer in one. If it comes out clean or with a few crumbs, it's fine. If it comes out sticky or a bit damp it needs a few more minutes.
Let them cool completely - then ice, decorate and enjoy these delicious, easy cookies!