Preheat the oven to 350 degrees F. Butter a glass 9-by-13-inch baking pan and line it with 2 overlapping pieces of parchment paper so the paper comes up over the edges of the pan. Butter the paper.
Make the brownies: Place a small pot of water over medium heat and bring to a simmer. Place a heatproof bowl over the simmering water, add the butter and chocolate and gently melt them together, stirring to combine. (This can also be done in the microwave; heat in 15-second intervals.)
Transfer the chocolate-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 prepared baking pan. Bake until a toothpick inserted into the center comes out somewhat clean, 25 to 30 minutes. Let the brownies cool completely in the pan.
Make the buttercream: Combine the cream cheese and butter in a stand mixer fitted with the paddle attachment and beat until smooth. Add the creme de menthe and beat until combined. Gradually add the confectioners' sugar and beat until smooth and fluffy.
Spread the buttercream over the brownies, then refrigerate until firm. Use the parchment to remove the brownies from the baking dish. Slice into bars.