Preheat the oven to 325°F. Line a 9x9-inch metal baking pan with parchment paper, leaving an overhang for easy removal. Lightly grease any exposed sides of the pan.
Add the granulated sugar and fresh mint to a small food processor. Pulse until the mint is finely chopped and evenly mixed with the sugar.
In a small bowl, whisk together the cocoa powder, whole wheat pastry flour, all-purpose flour, and salt.
In a microwave-safe bowl, combine the butter, chocolate, and milk. Microwave for 30–40 seconds, stirring until smooth. Set aside to cool slightly.
Beat the eggs on high speed for about 3 minutes until they become lighter in color and slightly thickened. Add the brown sugar, mint sugar, and vanilla, mixing for about 30 seconds to combine.
Mix in the melted chocolate mixture just until incorporated.
Fold the dry ingredients into the batter with a rubber spatula, mixing only until no dry streaks remain.
Spread the batter evenly into the prepared pan and bake for 37–42 minutes. The brownies are done when the edges are set but the center is still slightly soft. Avoid overbaking—carryover heat will finish baking them as they cool.
Let the brownies cool in the pan for about 5 minutes. Lift them out using the parchment paper and transfer to a wire rack. Enjoy warm for an extra fudgy brownie or cool completely before slicing.