Preheat the oven to 350°F/180°C and grease a mini muffin/cupcake pan with non-stick spray.
Prepare the brownie mix according to the instructions on the box and fill each cupcake cavity ⅔ full with the batter. Bake for 15-17 minutes until the edges are firm.
Remove the pan from the oven and use the back of a rounded spoon to indent the center of each brownie to create cups. Let the brownies cool in the pan for 10 minutes, then remove them to a wire rack to cool completely.
To Make Frosting:
Cream the butter on low speed in a stand mixer with a paddle attachment or a hand mixer until it is pale and creamy. Slowly add in 3 cups of powdered sugar and 3 tablespoons of heavy cream a little at a time, alternating between each. Mix on low for about 3 minutes. Add more sugar or more cream to get the frosting to the desired consistency.
Add vanilla, salt, and food coloring, and whip on high speed for two minutes until well combined.
To Decorate:
Fill a piping bag fitted with a large closed star tip with the frosting. Pipe the frosting into the cooled brownie cups, twisting back and forth to make a Christmas tree shape. Decorate with sprinkles.
Notes
Brownie Mix: Use your favorite. I used a Ghirardelli double chocolate mix which is 18 ounces.
Let the brownies cool completely. If you try to add icing to warm brownies, the frosting will melt.
Storing: These little brownies are delicate with their tall buttercream trees! Carefully place them into an airtight container and store them at room temperature. A cupcake carrier or cake carrier works really well to store these since they are tall like cupcakes. Eat within 5 days for best results.