Preheat oven to 350°. Grease a 13-by-9-by-2-inch pan.
Using an electric mixer, combine 2 sticks butter and sugar in a large mixing bowl; beat at medium speed until creamy. Add eggs, one at a time, beating well after each addition. Add 1 teaspoon vanilla, mixing well.
In a separate bowl, combine flour, 3/4 cup cocoa and baking powder. Gradually stir into butter mixture. Fold in mint chips. Spoon into greased pan. Bake for 35 minutes.
While brownies are baking, prepare frosting. Combine 1 stick butter, 1/3 cup cocoa, confectioner's sugar, 1 teaspoon vanilla and milk in a large mixing bowl; beat at medium speed with an electric mixer until dry ingredients are moistened. Beat at high speed until spreading consistency.
Remove pan from the oven and allow brownies to cool in pan on wire rack. Spread with chocolate frosting. Top with shaved mint thins.