Prep the pan: Heat oven to 350°F (175°C). Grease a 9x13-inch metal baking pan and line it with a parchment sling, leaving overhang on the long sides.

Make the batter: In a large bowl, whisk melted butter, granulated sugar, and brown sugar until glossy, about 30 seconds. Whisk in eggs and vanilla until smooth and slightly thickened.

Add dry ingredients: Sift in cocoa powder, flour, salt, and espresso powder. Fold gently with a spatula until just combined and no dry streaks remain. Stir in chocolate chips.

Bake: Spread the batter evenly in the pan and smooth the top. Bake 22 to 26 minutes, until the top is set and a toothpick near the center comes out with moist crumbs, not wet batter.

Cool completely: Set the pan on a rack and cool to room temperature, about 1 hour. For clean cuts, chill 30 minutes more in the fridge.

Make frosting: Beat softened butter with a mixer until creamy, 1 minute. Add powdered sugar, vanilla, and salt. Beat on low to combine, then on medium until fluffy, 2 minutes. Add milk 1 tablespoon at a time until thick yet pipeable. Tint with green gel food coloring.

Lift and trim: Use the parchment sling to lift the brownie slab onto a cutting board. Trim 1/4 inch from the edges for clean sides.

Cut into trees: Cut the slab into 3 long rows. Slice each row into rectangles about 2 to 2 1/2 inches wide, then cut each rectangle diagonally to form triangles. You should get about 16 trees, depending on size.

Decorate: Transfer frosting to a piping bag or a zip-top bag with a small corner snipped. Pipe zigzags across each triangle to look like garland. Add sprinkles and a star sprinkle at the top if you have them.

Add trunks and serve: Gently press a pretzel stick or mini candy cane stem into the base of each triangle for the trunk. Let the frosting set for 20 to 30 minutes before arranging on a platter.