Preheat the oven to 350 degrees F. Grease a glass 9-inch-by-13-inch baking dish with butter and line it with strips of parchment paper so the paper comes up over the edge of the dish. Grease the paper with more butter.
Place a small pot of water on medium heat, and bring to a simmer. Place a heatproof bowl over the simmering water and add the butter and chocolate to gently melt together, stirring to combine. (This can also be done in the microwave, heating at 15 second intervals.)
Transfer the chocolate and butter mixture to a large mixing bowl. Add the brown and granulated sugars, eggs, vanilla and salt and whisk together. Add the flour and gently whisk to combine, then pour into the baking dish. Bake until a toothpick inserted in the center comes out somewhat clean, 25 to 30 minutes. Let the brownies cool completely.
Carefully spread the Minty Buttercream over the brownies, then refrigerate until firm. Once the buttercream has firmed up, use the parchment sleeves to carefully remove the brownies from the baking dish. Slice into bars.
Combine the cream cheese and butter in the bowl of a stand mixer fitted with the paddle attachment and beat until smooth. Add the creme de menthe and beat until combined. Add the confectioners' sugar a quarter at a time and beat until the mixture is smooth and fluffy.