Preheat oven to 350ºF and grease mini muffin pan well with nonstick spray; set aside.
Prepare the brownie mix according to the box directions and fill each 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 measuring teaspoon to indent the center of each of the brownie cups. Let them cool in the pan for 10 minutes.
Use a paring knife or table knife to help pop them out of the pan and transfer them to a cooling rack to cool completely before frosting (about 30 minutes).
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 tablespoons 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 more powdered sugar. If your frosting needs to be thinned out, add more 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 with the frosting. Pipe the frosting onto the cooled brownie bites, twisting back and forth slightly to make a Christmas tree shape.
Top with sprinkles and star immediately before frosting sets. You will want to add sprinkles as you finish filling each bite since the frosting will start to set and make it hard for the sprinkles to stick.
I use Ghirardelli Double Chocolate Mix which is 18 ounces and makes about 36 bites. Any mix works but might make a little more or less.
Follow the package directions when making the brownies. Your package may call for a different amount of eggs, oil, and water.
Store in an airtight container for up to 5 days at room temperature or in the refrigerator.
An electric hand mixer may be used to make the frosting instead of a stand mixer.