In a bowl, cream butter and sugars together for a few minutes. Add egg then stir in the peppermint extract. Add flour, Dutch process cocoa powder, baking soda, and salt. Mix until just well combined.
Form into balls. Freeze for 20 min. Place on a lined baking sheet. Bake at 350° for 10-11 min. Let cool. Spread frosting on top then garnish with crushed peppermint candies.