Preheat oven to 350 degrees F. In a medium saucepan combine butter and unsweetened chocolate. Cook and stir over low heat until melted; cool.
Meanwhile, line a 9x9x2-inch or 8x8x2-inch baking pan with foil, extending the foil over edges of pan; set aside. Stir sugar into cooled chocolate mixture in saucepan. Add eggs, one at a time, beating with a wooden spoon after each addition just until combined. Stir in vanilla.
In a small bowl stir together flour and baking soda. Stir flour mixture into chocolate mixture just until combined. Spread half of the batter in the prepared baking pan; top with mint patties to within 1/2 inch of the edges. Carefully spread the remaining batter over patties.
Bake for 25 minutes for a 9-inch pan or 30 minutes for an 8-inch pan. Cool in pan on a wire rack. Using the edges of the foil, lift uncut brownies out of pan.
If desired, in a small saucepan cook and stir semisweet chocolate over low heat until melted. Drizzle over cooled brownies. Let stand until chocolate is set. Cut into brownies.