In a small saucepan, melt together the chocolate and butter, stirring occasionally. Allow to cool for 5 minutes.
In the meantime, beat the eggs and both sugars together in a large bowl until light in color, about 5 minutes. Beat in the peppermint extract and salt. Carefully fold in the melted chocolate mixture using a rubber spatula. Sift in the flour and baking powder and gently fold in too. Stir in the milk chocolate chips. Pour into a shallow baking dish and freeze for an hour or until firm.
When ready to bake, preheat the oven to 350°F (175°C) and line three baking sheets with parchment paper. Depending on what size cookies you want, scoop 2-3 tablespoon-sized balls onto the baking sheets (works great with an ice-cream scoop), set about 2 inches apart. Bake for 10-15 minutes or until the edges are looking dry and the tops are crackled. The cookies will still be very soft when finished baking!
Let the cookies cool on the baking sheet for 10 minutes or until they are firm enough to be transferred to a wire rack. Cool completely before drizzling with the melted white and dark chocolate and sprinkling with the crushed peppermint candies.
The cookies keep in an airtight container for up to four days.