Preheat the oven to 350ºF. Line an 8" or 9" square baking pan with foil and spray with nonstick spray. Set aside.
In a medium saucepan over medium heat, melt the butter and chopped chocolate, stirring frequently until completely smooth. Allow to cool for about 15 minutes.
Whisk sugar into cooled chocolate mixture until combined. Add the eggs one at a time, whisking after each addition until smooth. Whisk in the vanilla. Add the flour and salt and stir gently until completely combined.
Pour half of the batter into prepared pan. Layer unwrapped Peppermint Patties evenly over batter. Pour remaining batter over the Peppermint Patties, spreading with a spatula to insure all of the Patties are covered.
Bake brownies for 32-35 minutes or until a toothpick inserted in the center comes out mostly clean. Allow brownies to cool completely before cutting into squares. Warm brownies in the microwave to melt the peppermint filling before enjoying, if desired. Store brownies covered in the refrigerator up to 1 week or in the freezer up to 2 months.
*Room temperature eggs incorporate into batter much more easily than cold eggs, although this is not absolutely necessary.Adapted from homemade fudgy brownies