8 tablespoons (1 stick) unsalted butter, plus more for pan
8 ounces semisweet or bittersweet chocolate, chopped
1 cup sugar
3/4 teaspoon salt
3 large eggs
1/2 cup all-purpose flour
1/4 cup unsweetened cocoa powder
25 small (1-1/2 inch) peppermint patties
Preheat oven to 350°. Line an 8-inch square baking pan with aluminum foil, leaving an overhang on all sides; butter foil. Set aside.
Place chocolate and butter in a medium heatproof bowl set over (not in) a saucepan of gently simmering water. Stir occasionally just until melted, 4 to 5 minutes.
Remove from heat. Whisk in sugar and salt until smooth; whisk in eggs. Gently whisk in flour and cocoa powder just until smooth (do not overmix).
Spread 1/3 of batter in prepared pan. Arrange peppermint patties on batter in a single layer, leaving a narrow border on all sides. Top with remaining batter, and smooth surface. Bake until a toothpick inserted in center comes out with moist crumbs attached, 45 to 55 minutes.
Cool completely in pan. Use foil to lift from pan; peel off foil and discard. Cut into 16 squares.