In a medium mixing bowl, mix flour, baking powder, and salt. Set aside.
With an electric mixer, cream butter, and sugar. Add egg yolk, vanilla, and milk. Mix well. Add dry ingredients and beat until a soft dough forms. Stir in sprinkles. Place dough in the refrigerator for 30-60 minutes.
Preheat oven to 350 degrees F. Grease or place parchment paper on two baking sheets.
On a lightly floured surface, roll the dough out to 1/4 inch thick. Using a cookie cutter of choice, press out shapes in the dough. Place on baking sheets and bake for 10-12 minutes. Let cool completely.
Make the icing by adding all ingredients to a large mixing bowl and beat together with an electric mixer. Separate frosting into 2-3 different bowls.
Colour with food coloring of choice.
Place into a piping bag in equal portions. Pipe onto all cookies. Leave to set for 15-30 minutes.
Enjoy!!