Make the peppermint chips. Place chopped white chocolate and peppermint chocolate in a medium heatproof bowl and melt gently in the microwave or over a double boiler until just melted. Try not to over heat it - I pull it off the heat when there are still a few unmelted pieces left and let the residual heat melt them down with stirring. Stir in peppermint extract and pour the chocolate over a piece of parchment paper on a baking tray or a dish. Refrigerate until set and then chop into little pieces.
Make the cookie dough. Sift flour, cocoa powder and baking powder into a medium bowl. Whisk to blend evenly and set aside.
In another medium bowl, combine egg, sugar, vanilla and salt and whisk vigorously until pale and thick. Whisk in melted butter. Stir in flour mixture gently until mostly combined and then fold in chopped white chocolate. The dough will be very soft and sticky. Cover and refrigerate for 1-2 hours.
Use a 1 oz cookie scoop to portion dough and roll into balls. Roll dough balls in granulated sugar then roll generously in powdered sugar to coat evenly. Place them onto prepared baking sheets and bake for 8-10 minutes until cracked on the surface. Let cool on the trays and then enjoy!