Preheat oven to 350 degrees.
Place 1 cup (2 sticks) butter and chopped chocolate in a medium heavy-bottomed saucepan. Heat over low heat, stirring frequently, until both butter and chocolate are melted. Set aside and let cool for about 5 minutes.
Combine sugar and mint in a food processor and pulse until mint is finely chopped and incorporated into the sugar.
Place eggs and mint sugar in a large bowl and whisk together well.
Gradually whisk in chocolate/ butter mixture.
Whisk in flour, salt, and bourbon.
Pour into a lightly greased 9X13-inch pan. Bake 25-28 minutes. Let cool before frosting.
To make frosting, melt chocolate in a small, heavy bottomed saucepan over low heat. Let cool for 5-10 minutes.
Using an electric mixer, beat butter and sugar together. Add 3 tablespoons milk and beat, stopping to scrape down the sides, until smooth.
With mixer on low, slowly add chocolate and then beat on medium until mixed well.
Mix in vanilla extract and salt and add an extra tablespoon of milk if frosting is too thick.
Spread frosting on brownies.