Preheat the oven to 350°F. Grease a large baking pan (or two of the Temp-tations bakers) and set aside.
Combine the flour, cocoa powder, salt, and baking powder in a medium-size bowl. Lightly whisk to evenly combine all the ingredients. Set aside.
Melt the butter in a large saucepan. Stir in both sugars and the vanilla. Mix for about a minute. Remove from the heat and add the eggs. Mix until fully incorporated. Add the dry ingredients and whisk lightly until combined. Fold in the chopped mints and spread the batter into the prepared pan. Bake for 35-45 minutes, or until a knife inserted in the center just comes out clean.
While the cake is cooling, prepare the frosting. Cream the butter, mint extract, and half-and-half with a stand mixer, fitted with a paddle attachment, until smooth. Gradually add the confectioners' sugar and beat until light and creamy.
When the brownies have completely cooled, frost the brownies. If desired, spoon some store-bought chocolate icing into a piping bag and drizzle it on top of the white frosting.
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