Step 4: Fillem' Up!
Make some royal icing, any recipe will do, and you could even make half a recipe or something. You only need enough to outline the circles and "glue" them to the horn once it's filled with MnMs.
Put your candies of choice into the cookies, use the royal icing to secure a cap on the end of each one, then wait overnight for them to dry perfectly.
Once that's done, you can put your glitter layer on. Or you can do that before filling them, it's up to you lol.
For the glitter --- just use a food-only paint brush or your fingertip to paint the piping gel on in a very thin layer. Once you have your surface barely glossy, sprinkle with disco dust. For ease of use, put a mini cupcake wrapper over your disco dust, secure with mini rubber bands, poke holes into it and that will create a shaker. Then you can remove the wrapper to put the extra glitter in, then the wrapper back on, with mini rubber bands and screw the lid right on top.