To make the glaze, sift the icing sugar into a bowl. Whisk in the vanilla extract. Add the milk a half-teaspoon at a time, whisking between each addition, until the glaze reaches the desired consistency.
Divide between two bowls and tint half with green food colouring, and half with orange. Set aside.
Mummy Treats
Attach two candy eyes with dabs of icing. Use a piping bag fitted with a flat tip, and pipe strips of buttercream criss-crossing over top of the rice krispie treat. Don't cover the eyes.
Many-Eyed-Monster Treats
Spoon an uneven, drippy-looking layer of glaze on two-thirds of the treat. Top with several candy eyes.
Frankenstein Treats
Top with a layer of green glaze and add two eyes. Using black piping gel, pipe attached upside-down triangles at the top of the treat for hair.
Pipe a squiggly line for the mouth and an intersected line as stitches. Add silver sprinkles for staples, and mini peanut butter cups as neck bolts (attach with more glaze).
Jack-o'-Lantern Treats
Fully cover the top of the treat in orange glaze. Pipe a jack-o-lantern shape with the black piping gel and optionally add spooky eyes.
Notes
You’ll need a double batch of the rice krispie treats to make enough to frost with the different variations. Make them in a rectangular dish so that you can cut the longer rectangles rather than squares.