Preheat the oven to 350°F (180°C). Grease a 9 x 13–inch (22 x 33–cm) pan with butter or nonstick cooking spray and then line the pan with parchment paper, leaving an overhang on all sides. Grease the parchment with butter or nonstick cooking spray and then set it aside.
To a small saucepan, add approximately ½ cup (120 ml) of water and bring it to a boil. In a small bowl, add 1 tablespoon (15 ml) of the boiling water. Add the espresso powder and stir it to combine until a thick liquid or paste forms, then set it aside.
In a large bowl, whisk together the cocoa powder, dark brown sugar, and granulated sugar. Add the melted butter and espresso mixture and whisk until combined. Add the eggs, one at a time, making sure to beat the mixture in between each addition. Then stir in the vanilla and peppermint extracts.
Using a rubber spatula or wooden spoon, fold in the salt and flour until just combined, and then spread the batter evenly into the prepared pan. Bake for 25 to 30 minutes, sprinkling the crushed peppermint candy, if using, on top of the brownies about halfway through baking. Test the brownies with a toothpick inserted into the center of the pan. If it comes out with wet batter, the brownies are not done. If there are only a few moist crumbs, the brownies are done!
Remove the pan from the oven and place it on a wire rack. Allow the brownies to cool completely in the pan before cutting them into squares. Cover and store leftover brownies in an airtight container at room temperature for 3 to 4 days.