Cream the butter and sugar in a large bowl, then add the egg yolk and vanilla and briefly beat to combine. Sift over the flour and stir until the mixture is well combined, you'll probably need to finish it off with your hands to bind the mixture.
If you're adding bits, lessen the flour and add a drop of milk if needed. For chocolate ones, swap 50g flour for 50g cocoa powder.
Roll the mixture out and use cutters. Bake at 180 for about 8-12 mins depending on your oven.