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.