Line a 9" square pan with foil, allowing foil to extend over ends of pan; grease foil. Stir together first 3 ingredients; press into prepared pan. Bake at 350° for 10 minutes. Sprinkle chopped peppermint patties over warm crust.
While crust is baking, combine 1/2 cup butter and chocolate squares in a glass bowl. Microwave at HIGH 1 minute or until butter melts; stir until chocolate melts and mixture is smooth. Stir in sugar, eggs, vanilla, and peppermint extract. Add flour and salt; stir well. Pour over crust.
Bake at 350° for 33 minutes. Remove from oven, and immediately sprinkle with halved peppermint patties. Bake 2 1/2 more minutes. Remove from oven, and let cool 1 minute. Spread and swirl softened patties over brownies. (Don't wait too long before swirling or candy may be too firm to spread.) Cool completely in pan on a wire rack. Lift brownies out of pan; cut into squares.