Spread smoothly and evenly on top of the cooled brownies and return the brownies to the freezer while preparing the topping.
Place the mint chips in a large heat-safe bowl. Bring the heavy cream and butter *just* to a boil. Pour the heavy cream and butter mixture over the mint chips and stir gently until smooth.
Remove the brownies from the freezer and top with the melted mint chips mixture. Tip the brownie pan from side to side to get the chocolate to coat evenly.
*My ultimate favorite mix in this recipe is the Betty Crocker's Original Supreme mix (with packet of fudge in it) and another favorite is the Ghiradelli Chocolate Ultimate Fudge Brownie Mix.
**The video shows the topping being just butter + mint chips, but after making these so many times, I've changed the recipe to reflect a topping I like even better - the mint chips + butter + heavy cream!