In the bowl of a KitchenAid mixer, beat together the butter, granulated sugar, and vanilla extract until smooth.
Add the egg and combine.
Combine the flour, baking soda, baking powder, and salt together.
Slowly add the dry ingredients to the wet ingredients.
Beat until dough is combined.
Using a cookie scoop, drop the cookie dough onto a parchment lined cookie sheet.
Spray the bottom of a glass with non-stick cooking spray, and press each cookie until about ½ inch thick.
Bake for 7-8 minutes or until the centers are set.
Allow the cookies to cool completely before icing them.
Prepare the Frosting:
Whisk the powdered sugar, milk, and vanilla extract together. If you find the icing is too thick, add a little more milk. If it is too thin, add more powdered sugar.
Divide the icing into two small bowls. Add green food gel coloring to one bowl and red food gel coloring to the other bowl. Mix well until the desired color is reached.
Decorate the Cookies:
Dip the cooled cookies into the prepared icing.
Place the cookies on a cooling rack that's placed on top of a baking tray so the icing can drip off the cookies.
Before the icing hardens, sprinkle the crushed candies on top.
Notes
If you find the icing is too thick, add a little more milk. If it is too thin, add more powdered sugar.