Preheat the oven and prepare the brownie mix as directed on the box.

Spray a mini muffin pan with non-stick cooking spray. Fill each muffin well 2/3 full with brownie batter.

Bake for 10-15 minutes or until a toothpick inserted comes out clean.

Remove from the oven and use a 1/2 tablespoon to create indentations in each brownie. Allow the brownies to cool completely then remove from the pan.

To make the buttercream, beat the butter with an electric mixer until smooth, 30 seconds. Gradually add in the powered sugar, beating until incorporated. Add in a small amount of green food coloring, beat until mixed.

Use a spoon to put a dollop of buttercream in each brownie cauldron. Use the spoon to create wisps in the buttercream.

Add large green nonpareils and small green nonpareils to the buttercream. If you want to create the look that the cauldron is bubbling over, have the buttercream go down the side of the brownie and add nonpareils there too.

Add a broken pretzel stick to each cauldron as a stir spoon.

Serve or store on the counter until ready to serve if serving the same day, otherwise store in tupperware container until ready to serve.