Holiday sprinkles or crushed candy cane (optional)
01Preheat oven to 350 degrees F. Using a stand mixer on medium speed, mix together butter, egg, sugar and vanilla extract until smooth.
02In a separate, medium-size bowl, stir flour, cocoa powder, baking soda and salt together until ingredients are combined.
03Alternate mixing in the milk and dry ingredients into the butter mixture. Stir the mixture until a soft batter forms and no dry ingredients are visible.
04Scoop dough onto greased baking sheets with a spoon or large cookie scoop (about 2 inches across), placing them 2 inches apart. Bake for approximately 14 minutes; cookies are done when top springs back from your touch and bottoms are slightly browned. Place cookies on a rack and let them cool to room temperature.
05For the frosting, using a whisk and bowl or stand mixer on medium speed, blend together butter, powdered sugar, milk and vanilla or peppermint extract until mixture is fluffy and peaks when poked with whisk.(about 3 minutes)
06If desired, mix in a few drops of food coloring.
07To assemble, generously frost the bottom of one cookie, then top with a second cookie. Try to pair cookies of the same size. Repeat until all cookies are paired with frosting in the middle.
08Roll sides of each whoopie pie in sprinkles or crushed candy cane until all visible frosting is covered.
09Store in an airtight container. If storing longer than two days, place them in the refrigerator or freezer.