Pre-heat oven to 350 degrees and butter and line an 8x8 baking pan with foil, so it's overhanging on all sides. Butter foil as well.
Put butter and chocolate in a medium heatproof bowl set over a saucepan of gently simmering water (alternatively, use a double broiler). Stir butter/chocolate occasionally just until both are melted, about 4- 5 minutes. Remove from heat.
Whisk sugar and salt into the chocolate mixture until smooth. Whisk in eggs until fully combined. Gently whisk in flour and cocoa powder just until smooth, taking care not to over-mix.
Spread ⅓ of the chocolate batter in the bottom of prepared pan. Place peppermint patties on top of the batter in a single layer, leaving a small border on all sides. Pour the remaining batter over the top of the peppermint patties and smooth surface.
Bake for 38-42 minutes, until a toothpick inserted in center comes out clean/with moist crumbs attached.
Let brownies cool completely in pan. Once cool, use foil to lift out of pan and place on cutting board. Cut brownies into 16 squares.