Preheat your oven to 350°F (175°C). Line an 8x8-inch square baking pan with parchment paper, allowing some overhang for easy removal later. Lightly grease the parchment paper or pan to prevent sticking.
In a medium bowl, sift together the flour, cocoa powder, baking powder, and salt. Set aside.
In a large bowl, whisk together the sugar and oil until well combined. Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
Gradually add the dry ingredients to the wet ingredients, folding gently just until combined. Avoid overmixing to keep the brownies tender.
Pour the batter into the prepared pan and spread it evenly. Bake for 20 to 25 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs. Allow the brownies to cool completely in the pan.
In a medium bowl, beat the dairy-free margarine until creamy. Gradually add the powdered sugar and cocoa powder, mixing on low speed. Add water or plant-based milk tablespoon by tablespoon until you reach a spreadable consistency. Add green food coloring if desired and mix until the color is uniform.
Once the brownies are completely cool, lift them out using the parchment paper. Use a Christmas tree-shaped cookie cutter to cut out as many trees as possible.
Spread a thin layer of the chocolate frosting over each brownie tree. Decorate with your favorite holiday sprinkles, edible pearls, or colored sugar. Dust lightly with powdered sugar to create a snowy effect.
Arrange your Christmas Tree Brownies on a festive platter and watch them disappear quickly at your holiday gathering!