First, prepare your 9-inch by 13-inch baking pan. Spray it with non-stick spray or line it with heavy-duty aluminum foil. If you use foil, ensure it hangs over the edges for easy removal.
In a large bowl, measure out the Lucky Charms cereal. Set aside a quarter cup of the marshmallows from the cereal for later use.
Remove two cups of the mini marshmallows and set those aside as well.
Now, take a large microwave-safe container and add the butter slabs along with the remaining mini marshmallows. Microwave this mixture for about two minutes. After microwaving, stir until the butter and marshmallows are completely melted and combined.
Add in the vanilla and give it a good stir until it mixes into the melted marshmallow mixture.
Next, quickly mix the Lucky Charms cereal into the melted marshmallow blend. Stir thoroughly until the cereal is completely coated.
Incorporate the reserved mini marshmallows and stir gently. You want them to soften and partially melt, creating delightful pockets of marshmallow goo throughout the mixture.
Now, it’s time to press the mixture into the prepared pan. Butter your fingers to help with this step, preventing the mixture from sticking to you, or you can use the butter wrapper as a tool.
Once the mixture is evenly pressed into the pan, take the reserved Lucky Charms marshmallows and press them into the top for a fun, colorful finish. Sprinkle with pastel sprinkles if desired.
Allow the treats to cool at room temperature for at least an hour. If you used foil, lifting the treats onto a cutting board for slicing will be easy. Otherwise, cut them directly in the pan.
Store any leftovers in an airtight container to keep them fresh and chewy.
