Heat the oven to 350 degrees F and grease 9 x 9 inch glass baking dish with butter. Line it with parchment paper so that is overlaps the sides, and then grease the parchment paper as well.
In a large saucepan, melt the butter over medium low heat. Whisk in the cocoa powder until smooth, then stir in the dark chocolate chips and sugar. Remove from heat, and whisk in the salt, vanilla, and eggs. Let the batter cool slightly, and then stir in the mint chips (if using).
Pour the brownie batter into the prepared pan and bake in the oven for 20-25 minutes. When a toothpick inserted in the center comes out clean, remove the brownies from the oven and let cool. Lift the whole square out of the dish and chill in the freezer for 20-30 minutes. Flip the block of brownies over, so that the flaky top is now the bottom, and slice the brownies into 16 even squares. Chill the squares for another 20-30 minutes as you prepare the icing.