Preheat oven to 350ºF. Line an 8 or 9-inch baking pan with foil or parchment paper (let edges hang over to use as handles). Spray with nonstick spray.
In a medium-sized saucepan, melt butter over medium heat. Let butter cool slightly and whisk in eggs, egg yolk, vanilla extract, peppermint extract and milk. Mix in brownie mix, then fold in Peppermint Crunch Junior Mints® and white chocolate chips. Line an 8- or 9-inch baking pan with aluminum foil and spray with non-stick cooking spray. Scoop brownie batter into the pan and spread around evenly (wet a spatula to smooth top if needed).
Bake brownies in a 350°F oven for 18-20 minutes or until a toothpick inserted into the center comes out clean. Cool brownies completely and then refrigerate at least 30 minutes. Carefully lift chilled brownies from pan using the paper handles. Separate foil or paper from brownies and cut into 2-to 3-inch squares.
Brownies will keep in an airtight container at room temperature for up to 3 days.