Prep the pan and oven: Preheat your oven to 350°F (175°C). Line an 8x8-inch metal pan (for thicker brownies) or a 9x9-inch metal pan (for slightly thinner brownies) with parchment paper, leaving an overhang on two sides to lift the brownies out easily. For the smoothest edges, cut the parchment slightly narrower than the pan so it doesn’t wrinkle in the corners, and use a tiny dab of water or oil under the parchment to help it stay put.
Mix the dry ingredients: In a medium bowl, whisk together the flour (1 cup), cornstarch (1 teaspoon), salt (1/2 teaspoon), and baking soda (1/4 teaspoon) until everything is well combined and there are no streaks. Set this bowl aside while you make the chocolate base.
Make the chocolate base: In a medium saucepan, melt the butter (3/4 cup) over medium-low heat. Remove the pan from the heat and immediately whisk in the sugar (1 1/2 cups) and cocoa powder (3/4 cup) until the mixture is smooth and glossy. Let it cool for about 5 minutes, until the pan feels warm but not hot to the touch, so the eggs don’t scramble when you add them.
Finish the brownie batter: Whisk the eggs (2), extra egg yolk (1), and vanilla (1 tablespoon) into the cooled chocolate mixture, giving it a few vigorous strokes to create a shiny, smooth batter. Switch to a spatula and gently fold in the dry ingredients just until no dry flour streaks remain. If you’re using chocolate chips or chunks (3/4 cup), fold them in at the very end so you don’t overmix the batter.
Bake the brownies: Spread the batter evenly into the prepared pan and tap the pan lightly on the counter a few times to release any large air bubbles. Bake for 25–35 minutes, starting to check around 22 minutes (especially if using a 9x9 pan), until the top looks set and a toothpick inserted into the center comes out with just a few moist crumbs (not wet batter). If you like using an instant-read thermometer, the center should register about 190–200°F (88–93°C) when the brownies are done.
Cool, level (if needed), and cut into trees: Let the brownies cool completely in the pan on a wire rack, at least 1 hour. Use the parchment overhang to lift the slab carefully out of the pan. If the center sunk a bit, don’t worry—since you’re decorating these, you can gently level the top with a long serrated knife just like you would a cake. Use your Christmas tree triangle template to cut the brownies into tree shapes: lay the template over the brownies or mark the edges where the lines would fall, then slice into long strips and cut each strip into triangles. Save any scraps for brownie parfaits, ice cream sundaes, or hot chocolate toppings.
Make the frosting: In a large bowl, beat the softened butter (1/2 cup) with a hand mixer or stand mixer on medium speed until creamy and pale, about 2–3 minutes. Gradually add the powdered sugar (1 1/2 cups), about 1/2 cup at a time, mixing on low until each addition is incorporated so you don’t get a sugar cloud. Once all the sugar is in, add the vanilla (1 teaspoon), a pinch of salt (if using), and 1/2 tablespoon of cream or milk. Beat on medium until the frosting is smooth and fluffy. Add the green gel food coloring a drop at a time until you reach your desired tree green.
Adjust frosting consistency and (optional) two-tone effect: If the frosting feels too stiff to pipe, beat in additional cream or milk, a teaspoon at a time, just until it pipes smoothly and holds soft peaks. If it feels too soft and won’t hold its shape, beat in extra powdered sugar, 1 tablespoon at a time. For a snow-tipped effect, tint the main frosting green and leave a small portion white (or use canned white frosting), then spread each color in a long line on its own piece of plastic wrap, roll each into a log, twist the ends, and tuck both rolls side by side into a piping bag fitted with an elongated oval tip (like Wilton #44 or #104).
Decorate the Christmas tree brownies: Arrange the brownie triangles on a parchment-lined baking sheet or serving board. Holding the piping bag at the “top” of each tree, pipe a zigzag pattern of frosting from the tip to the base so it looks like layered branches. While the frosting is still soft, press a star-shaped sprinkle at the top of each tree and add round or disc sprinkles to mimic Christmas lights. If you’d like trunks, gently press a pretzel stick or straight candy cane piece into the base of each tree. Let the frosting set slightly, then serve and enjoy.