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.

In the upper bowl of a double boiler over medium low heat, combine the water, sugar, and corn syrup, whisking until smooth. Add the white chocolate wafers and stir continuously until smooth, and then slowly add in the crème de menthe slowly to achieve the right mint green color.

Remove the upper bowl from heat, and apply the icing to the chilled brownie bites one by one. You can either dip the tops of the brownie squares into the icing, or drizzle the icing over top of them with a spoon. Let the icing cool, and then spray the tops with the glitter spray or sprinkle with edible glitter.