Heat the oven to 350°F (175°C). Line a 13 x 9-inch baking pan with parchment paper, leaving enough paper overhanging the sides to lift out the cooled brownie slab.

Whisk the melted butter and granulated sugar together until smooth. Add the eggs and vanilla extract, then whisk until the mixture is evenly blended.

Stir in the cocoa powder, flour, and salt just until no dry streaks remain. Fold in the semisweet chocolate chips if using, taking care not to overmix the batter.

Spread the batter evenly in the prepared pan. Bake for about 25 to 30 minutes, until the center is set and a toothpick inserted in the middle comes out with a few moist crumbs.

Cool the brownies completely in the pan for at least 1 hour. For cleaner cuts, chill the cooled brownies for another 20 to 30 minutes before shaping them.

Lift the brownie slab from the pan using the parchment paper. Cut it into long strips or rows, then cut each row into triangular tree shapes; a 13 x 9-inch pan usually produces 12 to 16 trees, with some scraps.

Make the frosting by beating the softened butter until smooth. Gradually add the powdered sugar, milk or cream, and vanilla, then beat until creamy and tint the frosting green with gel food coloring.

Pipe green frosting in a zigzag pattern over each brownie triangle, moving continuously from the top point to the wide base. Insert a candy-cane piece into the bottom of each tree and finish with sprinkles or candy decorations.