Soften butter slightly, ensuring it does not become oily.

In a large bowl, stir together the softened butter, icing sugar, salt, nutmeg, and egg yolk using a wooden spoon until well combined.

Gradually add the flour, mixing a little at a time, until the mixture becomes too stiff to stir with a spoon.

Turn the dough onto a lightly floured board and knead gently, incorporating any loose flour until the dough just begins to crack.

Roll the dough out to approximately 1/4 inch thickness.

Cut the dough into desired shapes using cookie cutters or a knife.

Carefully place the cut shortbread cookies onto an ungreased cookie sheet.

If desired, sprinkle the dough with colored sugar or sprinkles before baking.

Bake in a preheated oven at 350°F (175°C) for about 10 minutes, or until the edges are delicately golden brown.

Remove from oven and let cool completely.