Mint Brownies with Buttercream Frosting

Name: Mint Brownies with Buttercream Frosting
Prep time: 10 min
Total time: 10 min
Cuisine: American
Ingredients: Nonstick spray, for greasing tin; 113 grams unsalted butter (1/2 cup; 1 stick; 4 ounces); 200 grams granulated sugar (1 cup); 75 grams cocoa powder (1/2 cup + 1 tablespoon; 2 2/3 ounces); 1/4 teaspoon kosher salt; 2 large eggs, at room temperature; 1 teaspoon vanilla extract; 2-3 drops peppermint oil*; 75 grams all-purpose flour (1/2 cup + 1 tablespoon; 2 2/3 ounces); 135 grams Andes Mints (28-count box) or mint dark chocolate, unwrapped, coarsely chopped, and divided into 2 equal portions**; 130 grams chopped dark chocolate (1 cup; 4 1/2 ounces); 113 grams unsalted butter, softened (1/2 cup; 1 stick; 4 ounces); 225 grams confectioner’s sugar (1 2/3 cups; 8 ounces), plus more to adjust consistency; 15 milliliters whole milk (1 tablespoon), plus more to adjust consistency; 3-4 drops peppermint oil (or to taste); 2-3 drops mint green gel food coloring (optional)