Preheat oven to 350 degrees F. Liberally grease a 13x9" baking pan with cooking spray; set aside. Meanwhile, in a large bowl, prepare the brownie batter according to package directions. Stir the hot fudge sauce and cinnamon into the brownie batter to combine.

Spread the batter into the prepared pan and bake according to package directions, about 20-23 minutes or until a toothpick inserted near the center comes out clean or with moist crumbs. Do not over-bake. Cool completely in the pan.

While brownies cool, prepare your frosting. In a small saucepan, melt the butter over medium-low heat. Allow the butter to melt completely, then carefully watch it during the browning process. Over the next 5-7 minutes, the butter will begin to sizzle, foam and pop, eventually turning an amber color. Watch it closely to prevent it from burning, until the butter is a deep amber color with brown flecks in it. Remove the pan from the heat and pour into a heat-proof bowl to cool completely.

Once butter has cooled, combine it with the Bailey's liqueur, milk or heavy cream, and powdered sugar, stirring vigorously until a thick yet spreadable frosting is achieved. You want it thicker than a glaze, but something you can spread rather than pour. Add more powdered sugar if necessary.

Spread the frosting over the cooled brownies. Allow the frosting to set, approx. 20 minutes or so at room temperature, before cutting into squares. Serve immediately.

**For an alcohol-free version, feel free to substitute the Bailey's Irish Cream Liqueur with Irish Cream-flavored coffee creamer instead. Bailey's makes a fabulous coffee creamer that tastes like Irish Cream but sans alcohol.**