Preheat the oven to 350° F. Line a cookie sheet with parchment paper or a silicone baking mat and set aside.
In the bowl of a stand mixer, cream together the butter, granulated sugar and powdered sugar until well combined.
Add the egg and vanilla, mixing until mostly combined.
Add the flour, baking powder and salt mixing until a soft dough forms.
Add the crushed soft peppermint candies and white chocolate chips, mixing until evenly distributed.
Portion out the dough into ⅓ cup portions (#12 cookie scoop) and arrange on the prepared cookie sheet.
Bake 6 on each cookie sheet.
Flatten each cookie just slightly before baking.
Bake at 350° F for 13-15 minutes or until the edges begin to turn golden brown and then cookies have flattened and then puffed back up.
Allow the cookies to cool on the cookie sheet. While the cookies are baking, make the dark chocolate ganache.