Brownies: Melt 1 cup of butter in a small saucepan over medium heat. Remove from heat and stir in the cocoa powder. Set aside.
In the bowl of a stand mixer, beat the eggs then gradually add the sugar.Add the vanilla and chocolate mixture, then stir until smooth.
In a separate bowl, whisk the flour, salt, and baking soda. Add the dry ingredients to the wet ingredients.
Pour the batter into a 9x13 baking dish that has been sprayed with nonstick spray or lined with parchment paper and bake at 350 degrees for 30 minutes or until a toothpick inserted comes out clean. Remove from the oven and cool completely before adding the frosting.
Mint Frosting: Beat the butter in a mixing bowl until light and fluffy. Mix in the powdered sugar, a little at a time. Add the peppermint extract, milk, and food coloring to the frosting and mix for 1-2 minutes. Frost the cooled brownies and place in the refrigerator to set for at least one hour.
Chocolate Layer: Melt the butter and chocolate chips in a medium saucepan over medium-low heat. When the chocolate has melted, pour it over the mint frosting layer and carefully spread with a knife or offset spatula. Place the brownies back in the refrigerator for at least one hour to overnight.
When the chocolate has set, remove and cut into bars.