In the bowl of a stand mixer, cream together the butter and sugar for 2 minutes on medium speed.
Add in the egg, peppermint extract, and 1 teaspoon Kelly green gel food coloring. Beat until thoroughly incorporated.
In a separate, medium-size bowl, mix together the flour, baking powder, and salt. Slowly add to the sugar mixture. Mix just until incorporated.
If you desire a deeper shade of green, add additional gel food coloring, mixing well to fully incorporate.
Divide the dough in half and wrap each piece tightly in plastic wrap. Refrigerate for at least 2 hours.
When ready to bake, preheat the oven to 350 degrees (F) and line baking sheets with parchment paper.
Sprinkle a flat surface or dough mat with flour and roll out a portion of the dough to about 1/8-inch to 1/4-inch thickness.
Using shamrock cookie cutters, cut the dough into shapes and place on the prepared baking sheets.
Refrigerate the shapes for 10 minutes, then place in the oven and bake for 8 to 10 minutes, depending on the thickness. The bottom edges of the cookies should just start to turn golden, although with the green coloring, it’s not that noticeable.
Remove cookies from the oven and allow to cool on the baking sheet for 5 minutes. Remove cookies from baking sheet and allow to completely cool on a wire rack.
Royal Icing:
Beat all ingredients until icing forms peaks (7-10 minutes at low speed with a heavy-duty mixer, 10-12 minutes at high speed with a hand-held mixer). Cover icing surface with a damp paper towel to keep from crusting over.
Putting it together:
Tint a portion of the royal icing a light green, if desired. Place the light green icing in a pastry bag fitted with a size 2 or 3 piping tip (or if you don’t have, you can always use a Ziplock bag and snip a tiny hole in the corner). Place some of the white royal icing in another pastry bag fitted with a sized 2 or 3 piping tip.
Decorate the shamrocks with the royal icing. If desired garnish with sprinkles.
Or, if you prefer to coat the shamrocks with a thin layer of icing for decorating with sprinkles only, thin a portion of royal icing with water until it is easily spreadable. Spread over the surface of the shamrock cookie and add sprinkles as desired.
Allow the cookies to completely dry then place in an airtight container with wax or parchment paper between the layers. Store at room temperature for up to 5 days.
Royal Icing tips:
It is imperative that your bowls, spoons, spatulas, beaters and anything else that comes in contact with this icing is grease free. I use Dawn Liquid Dish soap on these items because I know it will remove any trace of residual grease.
For stiffer icing, use 1 tablespoon less water.
Cover the icing bowl with a damp paper towel or plastic wrap in between working with it.
For longer storage, store in a Ziplock bag in the refrigerator. Icing will keep for one week.