01 - Preheat oven to 350°F and line a 9x9-inch baking pan with parchment paper, leaving an overhang for easy removal.
02 - Melt butter and semisweet chocolate together in a heatproof bowl over simmering water, stirring until smooth. Remove from heat and cool slightly.
03 - Whisk granulated and brown sugars into the melted mixture. Add eggs one by one, whisking well after each addition. Stir in vanilla extract.
04 - Sift flour, cocoa powder, and salt into the wet mixture. Fold gently until just combined, avoiding overmixing.
05 - Pour half the batter into the prepared pan and smooth the surface. Arrange peppermint patties evenly over the batter, leaving a small border.
06 - Pour remaining batter over patties, spreading gently to cover completely.
07 - Bake for 32 to 35 minutes until a toothpick inserted in the center comes out with a few moist crumbs.
08 - Allow brownies to cool completely in the pan before lifting out using parchment overhang and cutting into squares.