Combine butter, powdered sugar, peppermint extract, egg yolk and color paste in a mixing bowl. Beat on medium speed for about a minute. Stir in flour, baking powder and salt.

Roll dough into teaspoon size balls and place onto a baking sheet.

Bake for 10 minutes or until set and lightly browned on the bottom. Remove from oven and let set for 2 minutes. Place one Junior Mint candy on the cookie and gently press into the center.

Let cool completely before serving.