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

Melt the chopped chocolate and butter in a large microwave-safe bowl or double boiler, then stir until smooth and set aside to cool slightly.

In a separate bowl, whisk together the granulated sugar, eggs, and vanilla extract until light and slightly thickened.

Pour the cooled chocolate mixture into the egg mixture and whisk until well combined.

In a medium bowl, whisk together the flour, cocoa powder, baking powder, and salt. Gradually add these dry ingredients to the wet ingredients, mixing until just combined and avoiding overmixing.

Drop rounded tablespoons of dough onto the prepared baking sheets, spacing them apart, and bake for 10-12 minutes until edges are set but centers are still soft.

Cool the cookies on the baking sheets for a few minutes, then transfer them to a wire rack to cool completely.

While cookies cool, beat softened butter until creamy, then gradually add powdered sugar, milk, and peppermint extract to make the frosting; optionally, tint a small amount red with food coloring.

Once cookies are completely cool, spread or pipe the white peppermint frosting onto each. Swirl in red frosting for a festive look if desired, and sprinkle with crushed peppermint candies.