Separate the Lucky Charms mini marshmallows from the cereal.
In a large pot melt butter and marshmallows over medium heat until smooth and creamy. Stir in vanilla and cereal (not the mini marshmallows).
Press warm cereal mixture into the pan. Sprinkle immediately with mini marshmallows and gently press to adhere.
Cool completely and cut into squares.
Note: If you would like a thicker square you can use a smaller pan. If your squares are thick, press 1/2 of the cereal into the pan sprinkle with 1/2 of the mini marshmallows. Top with remaining cereal and marshmallows.