Prepare a 8x8 inch or 13x9 inch pan with shortening; set aside. In a large saucepan melt butter over low heat with the lemon zest and juice. Add in marshmallows and stir until completely melted. Slowly stir in the dry cake mix. Once combined remove from heat and stir in the cereal. Using greased spatula press mixture into greased pan. Cool on the countertop for approximately 30 minutes.