Preheat the oven to 325°F. Spray an 8x8-inch baking pan with nonstick spray. Line baking pan with parchment paper or foil and spray with nonstick spray again.

In a medium bowl, whisk together the flour, salt, and baking powder.

In a large microwave-safe bowl, microwave the chocolate and butter on high for 45 seconds, then stir and heat for 30 seconds more. Stir again, and, if necessary, repeat in 15-second increments until melted. Gradually whisk in the sugar. Add the eggs, one at a time, whisking after each addition until combined. Whisk in the vanilla and peppermint. Add the flour mixture in 3 additions, folding with a rubber spatula until the batter is smooth.

Spread the batter into the prepared pan. Bake until a toothpick or cake tester inserted into the center of the brownies comes out with a few moist crumbs attached, 20-25 minutes. Remove from the oven and immediately sprinkle brownies evenly with white chocolate chips. Return to the oven for 1 to 2 minutes, or until the chocolate chips are melted. Smooth chocolate with an offset spatula. Sprinkle with crushed candies and press lightly into the chocolate. Cool to room temperature. Chill the brownies in the refrigerator for at least 1 hour before carefully cutting into squares with a small sharp knife. Store the brownies in an airtight container in the refrigerator for up to 4 days.