Place softened butter into the bowl of an electric mixer fitted with the paddle attachment. Beat on high until light and fluffy, scraping the sides of the bowl as necessary.
Add mint jelly into butter and beat until well incorporated. Add the confectioners sugar ¼ cup at a time, mixing until fully incorporated after each addition. Add a dash of salt and the heavy cream or milk 1 tablespoon at a time until a thick, but spreadable consistency is reached.
Spread the mint frosting over the cooled brownies. Place in the refrigerator to stiffen.
Add the chocolate chips and heavy cream to a microwave safe bowl. Microwave in 30 second intervals, mixing well between each interval, until the chocolate has fully melted and the consistency is smooth and glossy. Cool slightly so it is still pourable but not hot to the touch.
When the frosting on the brownies has stiffened, spread the chocolate ganache over the frosting so it is completely covered. Tap the pan a few times on a hard surface to help evenly distribute the ganache.
Place the pan in refrigerator to cool and allow the ganache to firm. When ready to cut, remove from refrigerator and run a sharp knife under hot water before cutting. Clean the knife frequently to allow for clean cuts.