Melt the butter and mix in cocoa and sugar. Add eggs, flour, baking powder, and salt. Mix well. Add nuts, if using.
Pour batter into a greased 9 by 13 baking pan. Bake at 350 degrees for 25-35 minutes. Cool.
In a medium sized bowl, beat butter with a hand mixer or in a stand mixer. Add salt and powdered sugar. Beat until smooth and fluffy. Add mint extract and food coloring. Mix. Add milk a little at a time until the consistency is a little thinner than cake frosting.
Spread mint icing over brownies. Place brownies in the freezer for a short time to stiffen the icing. Alternatively, use store bought vanilla icing and add 1/2 tsp peppermint icing and the 1-2 drops green food coloring.
Using same bowl as the mint icing, beat the remaining butter with a mixer. Add salt, cocoa powder, and powdered sugar. Beat until smooth and fluffy. Add vanilla extract and add enough milk to thin to frosting consistency. Spread over chilled mint icing.
These freeze well. Cut into squares and lay on waxed paper. Place in a ziploc bag. To eat, remove desired number of squares and allow to sit for about 5 minutes before eating.The brownies can be made with or without nuts in the brownie batter.Just an extra, but chopped Andes mints on top of the chocolate frosting would be a fantastic addition!