Line an 8×8-inch square baking tin with parchment in a criss-cross pattern, allowing a 1-inch overhang on all sides.

Prepare brownie mix according to package directions. Allow brownies to cool completely, at least 6-8 hours, or overnight.

In a large bowl or the bowl of a stand mixer, whip softened butter on high speed until light and fluffy, about 30 seconds. Add the confectioner’s sugar, 1/2 cup at a time, alternating with the milk and mixing well after each addition. If necessary, add a bit more milk or confectioner’s sugar to adjust the consistency. (The buttercream should be thick rather than runny or watery, but still thin enough to pipe with ease.) Add peppermint oil to taste.

Scoop about 65 grams (1/4 cup) into 2 small bowls. Tint one bowl with red gel food coloring, and leave the other bowl white. Tint remaining buttercream with green gel food coloring.

Transfer to individual piping bags, fitted with decorative piping tips.

Lifting by the parchment, remove the baked and cooled brownies from the tin. Transfer to a cutting board. Line a tray with parchment or a silicone baking mat.

With a 3- or 4-inch tree shaped cookie cutter, cut the brownie slab into trees. Arrange on prepared tray.

Pipe buttercream as desired.

Decorate with Christmas sprinkles and edible glitter.

Serve immediately. Store leftover brownies in a single layer in a parchment-lined airtight container at room temperature for up to 2 days.