Line an 8x8 inch baking dish with parchment paper.

Place the butter and marshmallows in a large microwave-safe bowl. Microwave in 30 second increments, stirring after each, until the butter and marshmallows are melted and smooth. Stir in the chocolate until smooth. If necessary, you can microwave one more time, but you don’t want the chocolate to seize, so try to stir for a few minutes until it melts. Stir in the vanilla extract.

Fold the cereal and pretzels in with a large spatula. This will take a few minutes because it’s super sticky and you don’t want to totally crush all the cereal. Make sure it’s all coated evenly. Press the mixture in the lined baking dish and even out the top with a spatula. Refrigerate for 2 to 4 hours.

Remove the bars from the fridge and immediately cover the top with mini marshmallows. Heat the broiler in your over to high. Set the bars under the broiler for 1 to 2 minutes, watching the entire time, until they are golden and bubbly. Remove the dish from the oven. Use a sharp knife to cut the bars into squares. Some of the marshmallow might be gooey but slowly and gently slice through it. Serve the bars immediately!