Preheat your oven to 350°F (175°C) and line a 9x13-inch baking pan with parchment paper.

In a large bowl, mix together the brownie mix, vegetable oil, eggs, and lemon-lime soda until well combined. Pour the batter into the prepared pan and spread evenly.

Place the pan in the preheated oven and bake for 15-17 minutes, or until the edges are set and a toothpick inserted near the center comes out with just a few moist crumbs.

Remove the brownies from the oven and let them cool at room temperature for 15 minutes. Then, transfer them to the refrigerator to cool completely, about 1-2 hours.

Break off the curved tops of the candy canes, leaving the straight sections to use as 'tree trunks.' Set these aside.

In a medium bowl, beat together the powdered sugar, softened butter, and heavy cream until the mixture is smooth and fluffy. Add green food coloring until you achieve your desired shade of green.

Once the brownies are completely cool, lift them out of the pan using the parchment paper. Use a tree-shaped cookie cutter or a sharp knife to cut the brownies into tree shapes.

Insert the straight candy cane pieces into the bottom of each tree shape as trunks.

Fill a piping bag with the green frosting and decorate the brownies with swirls or zigzags. Add festive sprinkles, star candies, or mini chocolate candies as decorations.

Arrange your brownie Christmas trees on a platter and serve. Perfect for holiday parties or as a fun family activity!