Preheat your oven to 350°F (175°C). In a large mixing bowl, follow the directions on your chosen brownie mix, adding in the softened butter until smooth. Pour the mixture into a greased 9x13 inch baking pan, ensuring it's evenly distributed. Bake for 20-25 minutes.

Once baked, remove the pan from the oven and place it on a wire rack to cool. Allow the brownies to cool completely in the pan for at least 30 minutes.

In a large mixing bowl, combine powdered sugar, vanilla extract, heavy cream, and a pinch of kosher salt. Use a hand mixer to blend the mixture on low speed until it becomes smooth and creamy. Gradually add green food coloring, mixing until you've achieved your desired shade.

Using an offset spatula or a butter knife, spread a generous layer of the green frosting over the cooled brownies. Cut them into tree shapes or squares. Sprinkle the tops with crushed candy canes and colorful sprinkles.

Carefully slice the brownies into tree shapes or squares, arranging them on a festive plate for serving. Consider drizzling melted chocolate over the top before serving.