Grease a 9" x 9" pan with butter or nonstick cooking spray. Optionally, line with parchment paper.

Spray a large mixing bowl with cooking spray, add the marshmallows, and microwave in 30-second increments until melted and puffy. Alternatively, heat in a large saucepan over the stove.

Grease your spatula with cooking spray or oil.

Add the butter and salt to the melted marshmallows and mix until combined.

Reserve a few colorful marshmallows for topping, then gradually add the Lucky Charms and Rice Krispies, stirring to combine.

Pour the mixture into the prepared baking dish and press into an even layer with a spatula. Use cooking spray on your hands if needed.

Sprinkle the reserved Lucky Charms on top while the treats are still warm. Allow to rest until room temperature or freeze for 20 minutes to set.

Use a knife to release the edges of the treats, then remove from the pan and cut into 9 equal squares.

Store in an airtight container or wrap individually in plastic wrap.