Start by preheating your oven to 375°F so it's fully hot by the time your dough is ready. Line your baking sheet with parchment paper now so you're not scrambling later. Having everything set up before you mix keeps the whole process smooth and stress-free.
In a large bowl, combine the red velvet cake mix, melted butter, and eggs. Stir until everything comes together into a thick, workable dough. It will look slightly sticky at first, and that's completely normal at this stage.
Let the dough sit for about 10 minutes before shaping it. This short rest firms things up just enough to make rolling easier. While you wait, spread your cocoa powder onto a shallow plate for the next step.
Scoop a rounded tablespoon of dough and roll it into a ball between your palms. Roll each ball in the cocoa powder until fully coated, then place it on your lined baking sheet. Space them a couple of inches apart since they'll spread slightly.
Bake for 12 to 14 minutes, watching for the cracks to form on top — that's the cauldron effect you want. The cookies should look set on the outside but still soft in the center when you pull them out.
Let the cookies cool completely on the pan before moving them to a rack. Once cooled, pipe on small dots of vanilla icing and press candy eyes into the cracks for that bubbling potion look.