Preheat your oven to 350 degrees F. Grease the cups of a mini muffin tin with non-stick cooking spray. Set aside.
Prepare the brownie mix according to the package directions. Fill each mini muffin tin cup 2/3 full with brownie batter.
Bake for 15-17 minutes until the edges are firm and the centers are still soft (but not gooey).
Remove the muffin tin from the oven. Use the back of a rounded teaspoon measuring spoon to indent the center of each brownie to make a cup shape. Cool the brownies in the tin for 10 minutes before transferring to a wire rack to cool completely.
While the brownies cool, beat the butter in a large mixing bowl on low speed until pale and creamy, about 2 minutes.
Add the powdered sugar 1 cup at a time, alternating with 1 tablespoon of heavy cream, while mixing over 3 minutes. (see note)
Add the vanilla, salt, and food coloring and mix. Add more food coloring as needed until your desired color is reached. Beat the frosting on high for 2 minutes. Transfer the frosting into a piping bag fitted with a large closed star tip.
Pipe the frosting into the brownie cups, twisting back and forth slightly to make a Christmas tree shape. Top with sprinkles. (You'll want to add the sprinkles quickly since the frosting will start to crust fast making it hard for the sprinkles to stick.)
Notes
If the frosting is too stiff, add more heavy cream (a little at a time) until the consistency is to your liking. If the frosting is too thin, add more powdered sugar until it firms up.
It's best not to let the brownie sit out in an overly warm environment or the frosting may start to melt. You can chill them in the fridge (if making them ahead of time) and then put the brownies out to come up to room temperature on the counter before serving.
Store any leftovers in an airtight container for up to 5 days at room temperature.