1 1/2 cups butter, softened
4 cups powdered sugar
2 tablespoons milk
1/4-1/2 teaspoon pure peppermint oil
a few drops of green food coloring (optional)
Beat butter at medium speed with an electric mixer until creamy. Gradually add powdered sugar beating until light and fluffy. Add milk, oil and food coloring (if using), and beat until you reach spreading consistency. Add more milk as needed in teaspoonfuls.
Spread on the cooled cake brownies and put in the fridge to harden the frosting up so you can easily spread the fudge frosting below.
No-Cook Fudge Frosting
4 3/4 cups powdered sugar
1/2 cup unsweetened cocoa powder
1/2 cup butter, softened
1/3 cup boiling water (may need a bit more to reach spreading consistency)
1 teaspoon vanilla
Combine powdered sugar and cocoa powder. Add butter, boiling water and vanilla. Beat with an electric mixer on low speed till combined. Beat for 1 minute on medium speed. Cool for 20-30 minutes or until it reaches spreading consistency.
The longer you beat the frosting, the less fudgy it will become so add enough water early on that it will be easy to spread over the mint frosting.