01 - Preheat oven to 350°F. Line a 9x9-inch baking pan with parchment paper, allowing 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 in both sugars until incorporated. Add eggs one at a time, whisking thoroughly after each addition. Stir in vanilla extract.
04 - Sift flour, cocoa powder, and salt into the wet mixture. Fold gently until just combined to avoid overmixing.
05 - Pour half the batter into the prepared pan and smooth evenly. Arrange peppermint patties in a single layer, leaving a small border at the edges.
06 - Pour remaining batter over the peppermint layer, spreading carefully to cover completely.
07 - Bake for 32 to 35 minutes, or until a toothpick inserted in the center comes out with moist crumbs, taking care not to overbake.
08 - Allow brownies to cool fully in the pan before lifting out and slicing into 16 squares.