Preheat the oven to 350°F and grease a mini cupcake pan well with nonstick spray and set aside.
Prepare the brownie mix according to the box directions and fill each cupcake cavity about ⅔ full with brownie batter.
Bake for about 15 to 17 minutes until the edges are firm but the center is still slightly soft (but not gooey).
Remove from the oven and use the back of a round teaspoon to indent the center of each of the brownie cups. Let them cool in the pan for 10 minutes.
Use a paring knife to help pop them out of the pan and transfer them to a cooling rack to cool completely before frosting.
While they are cooling, cream the butter on low speed with a paddle attachment until pale and creamy.
Slowly add in 3 cups of powdered sugar and 3 tbsp heavy cream a little at a time, alternating between each, while mixing on low over the course of about 3 minutes.
If your frosting needs a more stiff consistency, add remaining sugar. If your frosting needs to be thinned out, add the remaining cream.
Add vanilla, salt, and food coloring and whip on high speed for 2 minutes.
Fill a piping bag fitted with a large star tip (I used this one: https://amzn.to/3njvNfX) with the frosting.
Pipe the frosting onto the cooled cupcakes, twisting back and forth slightly to make a Christmas tree shape.
Top with sprinkles.