Preheat oven to 350 degrees.
Line an 8-inch square baking pan with parchment paper making a sling so the ends hang over the side. Fit another piece parchment going in the opposite direction so the entire pan is lined. Lightly spray the parchment with vegetable baking spray. Set aside.
In a small microwave safe bowl, combine the butter and chopped bittersweet chocolate. Microwave on 50% power for 2 to 3 minutes, stirring halfway through. Stir until all chocolate is melted then add the espresso powder. Set aside to cool slightly.
In a medium mixing bowl whisk together the sugar, eggs, vanilla, baking powder and salt until combined. Add the melted chocolate mixture and whisk until incorporated. Using a rubber spatula, stir in the flour just until combined. Pour the batter into the prepared pan, smoothing the top.
Bake for 30-35 minutes or until a toothpick inserted in the center comes out with moist crumbs, but no raw batter.
Place the pan on a rack and immediately sprinkle the chopped white chocolate all over the top. Allow the chocolate to soften a few minutes then spread with an offset spatula. Immediately sprinkle the crushed peppermint over the white chocolate.
Cool the brownies for at least 2 hours then using the parchment paper sling, remove the brownies from the pan. Cut into 16 pieces before serving.