Preheat the oven to 350°F (175°C) and lightly grease three 12-count mini muffin pans.
Mix Devil’s Food cake mix, eggs, oil, and black food coloring (if using) in a large bowl until a thick dough forms.
Scoop ½ tablespoon of dough into each muffin cavity and flatten slightly.
Bake for 8–10 minutes until edges are set and tops no longer look wet.
While warm, press an indentation into each cookie with the back of a 1-tablespoon spoon, then cool completely on a rack.
Beat butter until creamy, then mix in powdered sugar, vanilla, and salt. Add milk gradually until smooth and pipeable, then tint green for a “potion” look.
Pipe frosting into cooled cookie cups to resemble bubbling cauldrons. Decorate with green sprinkles, Halloween sprinkles, candy eyeballs, and a halved pretzel stick as a stir stick.
Serve at room temperature for the best texture.