Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.

In a mixing bowl, whisk together the all-purpose flour, cocoa powder, baking powder, and salt. Set this dry mixture aside.

In a separate bowl, cream together the softened butter and granulated sugar until light and fluffy, about 3-4 minutes.

Add the egg, vanilla extract, and peppermint extract to the butter and sugar mixture, mixing until combined.

Gradually add the dry mixture to the wet mixture, mixing until a dough forms. Fold in the crushed peppermint candies.

Roll out the dough to about ¼ inch thick and use a tree-shaped cookie cutter to cut out tree shapes. Re-roll any scraps until all dough is used.

Place the cookie trees on the prepared baking sheet, leaving space between them. Bake for 10-15 minutes or until set.

Let the cookies cool completely on a cooling rack.

In a small bowl, mix together the powdered sugar, milk, and green food coloring (if using) to create frosting. Adjust consistency with more powdered sugar or milk as needed.

Once the cookie trees are cool, decorate them with the frosting using a piping bag, and sprinkle additional crushed peppermint on top.

Allow the decoration to set before serving.