Lightly butter a 9x9 pan or line with parchment paper.
Add butter to a large saucepan over low heat and let it melt.
Add the 10-oz bag (roughly 6 cups) of mini marshmallows to saucepan and cook, stirring frequently, until marshmallows are mostly melted. Remove from heat while a few small lumps remain.
Stir in vanilla extract, milk powder (if using), and salt until smooth.
Fold in the Rice Krispies and the Lucky Charms cereal pieces only (not cereal marshmallows). Stir mixture gently until evenly coated in marshmallow.
Fold in additional 1 cup mini marshmallows and ⅓ cup Lucky Charms marshmallows
Immediately put mixture into the bottom of your prepared pan and gently press into an even layer (it can help if you lightly butter or oil fingers or spatula for pressing). Don't press too firmly or treats will get hard. Sprinkle the remaining Lucky Charms marshmallows over the top and press lightly to stick.
Let Lucky Charms Rice Krispies treats cool at room temperature for at least 30 minutes before slicing.
Store the cooled treats in an airtight container at room temperature for about a week.