Preheat the oven to 350 degrees F. Line an 8 or 9 inch square baking pan with aluminum foil sprayed with the cooking spray or, alternatively, you can use parchment paper.
Melt butter in a medium sized saucepan over medium heat. Allow to cool slightly. Add the sugar, eggs, vanilla, and peppermint extract to the melted butter; stir to combine. Add flour, hot chocolate mix, salt and baking powder to the mixture; stir until just combined. Spread the mixture in the prepared baking pan and, if desired, add about half of the Junior Mint candies to the batter.
Bake until the brownies begin to pull away from the edges of the pan and are set in the center, about 22-25 minutes. Remove from the oven and let cool completely on a rack. Add remaining candies to the top, if desired. Cut into 16 squares.