Preheat the oven to 350°F (175°C) and lightly grease several mini muffin pans.
Mix the cake mix, eggs, oil, and optional black food coloring until a soft dough forms.
Scoop small portions of dough into each muffin cavity and gently flatten the tops.
Bake until the cookie cups are set around the edges.
While still warm, immediately press the center of each cookie using the back of a measuring spoon or similar rounded tool to create a deep well.
Let the cookie cups cool completely before decorating.
Beat the butter until creamy. Gradually mix in powdered sugar, vanilla, salt, and enough milk to create a smooth piping consistency.
Tint the frosting bright green.
Pipe the buttercream into each cookie cup to resemble bubbling potion.
Decorate with green sprinkles, Halloween candies, candy eyes, and a small pretzel stick for the finishing touch.