Prepare your baking dish by greasing it lightly or lining with parchment paper.
In a large pot, melt the butter over low heat, stirring occasionally to prevent it from browning.
Add the mini marshmallows to the pot and stir constantly until they are completely melted.
Remove the pot from heat and stir in the vanilla extract until fully combined.
Fold in the Rice Krispies cereal until uniformly coated in the marshmallow mixture.
Finally, gently mix in the Lucky Charms marshmallows, taking care not to crush them too much.
Transfer the mixture into the prepared baking dish and press it down evenly with a spatula or your hands that are slightly greased.
Allow the treats to cool completely at room temperature, about 30 minutes, before cutting into squares.