Preheat your oven to 350°F (175°C) to ensure even baking.
In a double boiler or microwave-safe bowl, melt together the unsweetened chocolate and unsalted butter until smooth and glossy. Allow to cool slightly.
Add the packed light brown sugar to the melted chocolate mixture and mix until well combined. Add the eggs one at a time, followed by the vanilla extract, whisking until smooth.
Sift together the flour, baking powder, and kosher salt in another bowl. Gradually add this dry mixture to the chocolate mixture, stirring until just combined.
Pour the brownie batter into a lined 9x13 inch baking pan and spread it evenly. Bake for about 25-30 minutes, or until a toothpick comes out with a few moist crumbs. Let cool completely on a wire rack.
In a mixing bowl, combine powdered sugar, softened unsalted butter, heavy cream, mint extract, and green food coloring (if desired). Mix until smooth and fluffy.
Once cooled, spread the mint frosting evenly over the brownies using a spatula.
Melt the semi-sweet chocolate with the unsalted butter in a small saucepan or microwave. Stir until smooth and drizzle over the frosted brownies.
Refrigerate for about 30 minutes to set the frosting and chocolate drizzle. Cut into squares and serve!