Luck O' the Irish Brownie

INGREDIENTS

• 4 (1 ounce) squares unsweetened chocolate
• 1 cup margarine, softened
• 2 cups white sugar
• 3 eggs
• 2 1/2 teaspoons vanilla extract
• 1 cup all-purpose flour
•  
• 1/2 cup butter, softened
• 4 cups confectioners' sugar
• 2 tablespoons milk, or as needed
• 1 1/2 teaspoons peppermint extract
• 6 drops green food coloring
• 1 1/3 cups semisweet chocolate chips
• 6 tablespoons butter
• 1 tablespoon vanilla extract