Preheat your oven: Set it to 350°F (175°C) and line an 8×8-inch baking pan with parchment paper.
Mix the dry ingredients: In a large bowl, whisk together the oat flour, cocoa powder, Chocolate Thin Mint Whey Protein, baking powder, and salt.
Combine the wet ingredients: In a separate bowl, mix the applesauce, coconut sugar, eggs, melted coconut oil, vanilla extract, and peppermint extract until smooth.
Blend together: Gradually add the wet ingredients to the dry ingredients, stirring until well combined. If using, fold in the dark chocolate chips.
Bake: Pour the batter into the prepared pan and spread it evenly. Bake for 18–20 minutes or until a toothpick inserted into the center comes out slightly fudgy.
Cool and top: Let the brownies cool completely. Spread melted dark chocolate on top and sprinkle with crushed peppermint candies for a festive finish.