Prepare brownie mix according to package directions. (Be sure to line your tin with parchment, which will make it much easier to transfer the baked brownies to a cutting board.) Allow brownies to cool completely, at least 6-8 hours or overnight.

Buttercream:

Sift the confectioner’s sugar to remove any lumps.

In a large bowl or the bowl of a stand mixer, whip the softened butter on high speed until light and fluffy.

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.) Mix in the vanilla extract.

Divide into four portions.

Tint three bowls with gel food coloring (I chose pink, blue, and yellow, but feel free to use any colors you’d like!), and leave the other bowl white.

Decoration:

Line a tray with parchment, wax paper, or a silicone baking mat.

Fit three disposable piping bags with tiny (approximately 1/4-inch) star piping tips. (You can use open or closed star, or a mix of both!) Fill with the colored buttercream.  Fit a fourth bag with a tiny (about 1/8-inch) round writing tip, and fill with the white buttercream.

Lifting by the parchment, remove the cooled brownie slab from the tin. Place on a large cutting board.

With a 2 x 3-inch egg shaped cookie cutter (I used the egg from this set of Easter cookie cutters), cut the slab into eggs.

Transfer to prepared tray.

Using an up-and-down motion to create small individual stars, pipe desired patterns with the colored buttercream. (Feel free to copy my examples, or design your own!) Stripes, zigzags, dots, and diagonal lines are all easy to pipe.

With the white buttercream, add details such as outlines, swirls, dots, and borders. If desired, you can also add simple text to your eggs.

Serve immediately.

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