Using a stand mixer with the paddle attachment, cream together butter and sugar. Add egg and vanilla. Mix at low speed until fully blended. Then add flour, 1 cup at a time, starting on low speed and then increasing if necessary. Roll out to 1/4" thickness. Use a Christmas Tree cookie cutter to cut out the trees. Use a smaller tree cutter to cut out the middle. Place parchment or Silpat mats on your baking sheets.Bake at 350 degrees for 8 minutes. and remove from oven.Crush the peppermint candies by placing them in a zip bag and hitting the bag with a rolling pin until crushed. Sprinkle crushed candy in the middle of each cookie. Bake for an additional 2-3 minutes until the candy has melted and cookies have just started to turn golden brown around the edges. Cool completely in the pan before removing the cookies.
Frosting
In stand mixer, whip butter, vanilla and salt. Add powdered sugar and mix more. Add a few drops of food gel until you get your desired color. Add to a piping bag with a small star tip and pipe "pine leaves" over the cookies.
