Step 1

Melt the butter and chopped chocolate together in a large microwave-safe bowl by heating in 30 second increments, stirring between each burst of heat, until melted and smooth.  Allow the mixture to cool slightly for 10 minutes.

Step 2

Heat oven to 350 degrees.  Line a 9x13 baking pan with a parchment paper sling.

Step 3

Whisk both sugars into the cooled chocolate and butter mixture.  Add the eggs, one at a time, whisking after each addition until smooth.  Add the vanilla, then add the salt, flour, and cocoa powder at the same time and gently fold them in using a rubber spatula or wooden spoon, until no streaks of flour or cocoa powder remain.

Step 4

Pour the brownie batter into the prepared baking pan and bake for 32-36 minutes, just until the brownies start to pull away from the edges of the pan and a toothpick inserted into the center of the brownies comes out with only a few moist crumbs clinging to it.

Step 5

Remove from oven and cool completely in the pan before transferring them to a baking sheet using the parchment paper sling for easy removal.

Step 6

In a medium bowl using a handheld mixer, beat the butter until smooth and creamy.  Add the salt, powdered sugar, and milk and beat for 2 minutes on low speed, then add peppermint extract and food coloring and beat on high for 1 minute.  Taste and add more peppermint extract or green food coloring, if desired.

Step 7

Frost the completely cooled brownies and transfer them to the freezer while you make the chocolate frosting to help the mint frosting "set" on top of the brownies.

Step 8

In a medium bowl using a handheld mixer, beat the butter until smooth and creamy. Add the salt, powdered sugar, cocoa powder, milk, and vanilla, then beat for 2 minutes on low speed.  Increase the speed to high and beat for 1 minute more.

Step 9

Gently spread the chocolate frosting over the chilled layer of mint frosting.  Cut into squares and serve.