Grease a 9 x 13" pan. In a large pot, melt butter. Add marshmallows. Stir until melted. Add Trix. Mix until Trix is coated with marshmallow mixture. Press into prepared pan using a plastic glove which has been sprayed with Pam (or greased). Cool completely. Cut into 24 pieces using a teflon or greased knife.
*I've noticed that marshmallows come in bags ranging from 10 oz to 1 lb. If using a 10 oz bag, use 9 c of Trix. Treats will be slightly shorter, but barely noticeable.