Bake brownies according to the recipe and cool completely. Cut brownies into squares and place on a cutting board or counter where you can decorate.
While the brownies bake or cool, make your buttercream. If using store-bought frosting, empty the frosting into a bowl and whisk it to add air and volume. Stir in ¼ tsp of cinnamon for worn-looking bandages, if desired. If buttercream or frosting is too soft to hold its shape, chill it for 10 minutes. If it’s too stiff to pipe, add a little milk one teaspoon at a time.
If desired, make spooky eyes by drawing squiggly lines with a red food-safe marker. Set aside.
Get the pastry bag ready by cutting a small section off the bottom of the bag. If using a coupler, place it inside the pastry bag first, then attach the tip. If you’re not using a tip, cut a small hole in the bottom of the bag and pipe thicker lines.
Place the pastry bag in a small cup and fold the top over the rim to hold it open. Scoop in some buttercream, lift the bag out of the cup and twist the top to keep the buttercream from escaping.
Pipe the bandages by starting at the top of each brownie and working your way down, overlapping the lines a little so the brownie looks wrapped like a mummy.
Add the spooky eyes by pressing small or large candy eyes into the buttercream for added dimension. For a bloody mummy, add a few drops of red piping gel to the tops of some bandages.