Preheat the oven to 350 degree F and lightly grease three 12-count mini muffin pans.

In a large bowl, mix the cake mix, eggs, canola oil, and black food coloring (if using) until well combined.

Scoop about 1/2 tablespoon of dough and place in the mini muffin pans. Flatten the dough with your fingers.

Bake for 8-10 minutes or until the edges are set. Remove from the oven and immediately press an indentation into each warm cookie using the back of a 1 tablespoon measuring spoon. Set aside to cool completely.

While the cookie cups cool, make the buttercream. In a medium bowl, beat the butter until creamy. Gradually add powdered sugar, vanilla, and salt, beating until smooth. Add milk a little at a time until the buttercream is smooth and pipeable. Mix in green food coloring until the desired shade is reached.

Pipe the buttercream into the cooled cookie cups, filling each to resemble a bubbling potion.

Decorate with green sprinkles, candy eyeballs, and Halloween sprinkles. Break pretzel sticks in half and insert one into each cookie cup as a stir stick.

Serve and enjoy!