Add the Rice Chex cereal to a large mixing bowl and set aside.

Place the milk chocolate chips, creamy peanut butter, and butter in a small saucepan.

Heat the mixture over low heat, stirring constantly, until completely melted and smooth.

Remove the saucepan from the heat and stir in the vanilla bean paste until combined.

Pour the warm chocolate-peanut butter mixture over the Rice Chex cereal.

Gently fold the cereal with a spatula until all of the cereal is evenly coated.

Transfer the coated cereal to a large gallon-size zip-top bag.

Add the powdered sugar to the bag, seal tightly, and shake until all the cereal pieces are well coated.

Spread the coated cereal onto a baking sheet to cool and dry for several minutes.

Once cooled, transfer the snack mix to a serving bowl and sprinkle the Lucky Charms vanilla marshmallows evenly over the top.

Serve immediately or store in an airtight container until ready to serve.