Preheat oven to 350°F. Spray an 8x8-inch baking dish evenly with nonstick spray. Set aside.
In a large microwave-safe bowl add the butter and bittersweet chocolate chunks. Microwave in 15-30-second intervals for about 1 minute or until melted, stirring occasionally.
½ cup unsalted butter, ½ cup bittersweet chocolate chunks
Remove from the microwave and gradually add unsweetened cocoa powder, stirring as you go.
½ cup unsweetened cocoa powder
Add the brown sugar and all-purpose flour. Mix well.
1½ cups brown sugar, ¾ cup all-purpose flour
Add the vanilla extract, peppermint extract, eggs, and salt to the bowl. Mix together until everything is smooth.
3 large eggs, 1 tablespoon pure vanilla extract, 1 tablespoon peppermint extract, ½ teaspoon kosher salt
Fold in the mini chocolate chips.
½ cup mini chocolate chips
Pour brownie batter into the greased dish. Bake for about 25 minutes, turning the dish around halfway through cooking time. Brownies are done when an inserted toothpick cones out mostly clean. They will be a little fudgey and you don’t want to over cook them.
Allow brownies to cool fully before frosting.