Coat the bottom and sides of a 9×13-inch baking dish with butter. Side Note: For easier removal, you can line the dish with parchment paper if desired.
Melt the butter in a large saucepan over medium heat.
Remove ½ cup of mini marshmallows from the bag and set aside until later.
Add the remaining marshmallows to the melted butter, stirring constantly until melted and smooth, about 3-4 minutes. Side Note: You may need to put the large saucepan over low heat to help the marshmallows melt.
Once melted, remove from the heat, add the vanilla extract, and stir to mix well.
Immediately add the rice krispies and stir until well combined.
Add the ½ cup of mini marshmallows and most of the mini Cadbury eggs and stir gently to combine.
Immediately spread the mixture in the prepared baking dish, pressing it firmly into an even layer, using parchment paper to protect your hands, if desired.
Gently press the few remaining mini Cadbury eggs evenly on top.