Preheat your oven to 350°F (175°C). Line the baking dish with parchment paper, leaving some overhang for easy removal later.
In a mixing bowl, combine the granulated sugar, brown sugar, and melted butter. Whisk until smooth and creamy.
Add in the eggs, vanilla extract, and peppermint extract. Continue whisking until the mixture is fully combined and smooth.
In another bowl, sift together the all-purpose flour, cocoa powder, and salt. Gradually add the dry ingredients into the wet mixture, stirring gently using a rubber spatula until just combined.
Fold in the semi-sweet chocolate chips until evenly distributed.
Pour the batter into the prepared baking dish, spreading it evenly. Bake for 25 minutes or until a toothpick inserted into the center comes out with a few moist crumbs.
If desired, melt the white chocolate chips in a microwave or double boiler and drizzle over the warm brownies for added flavor and decoration.
Allow the brownies to cool in the pan for about 10 minutes, then use the parchment overhang to lift them out. Let them cool completely on a wire rack.
Once cooled, cut into 12 squares. Garnish with crushed peppermint candies on top.