Preheat oven to 350 degrees. Beat butter with sugar, egg and vanilla in a large bowl. In another bowl, combine flour, baking soda and salt. Add dry mixture and milk to the butter and sugar mixture, mixing until well blended.
Shape the dough into 1 inch balls.
Roll the balls in the red and green sugar.
Bake at 350 degrees on a greased cookie sheet for 8-10 minutes.
Remove from the oven and let cool for 2-3 minutes. Place an unwrapped Peppermint Bark Bell in the center of each cookie and push it into place.
Place cookies on a cooling rack and allow cookie to cool completely before serving.