Melt butter in a large pot over low heat until melted. Add marshmallows and stir until marshmallows are melted. Remove from heat.
Add Rice Krispy cereal, one cup at a time, stirring until well coated (I use a spatula sprayed with cooking spray to prevent sticking)
Press mixture into 13x9x2 inch pan lined with parchment paper and coated with cooking spray (dairy free). Press down into pan. Cool completely. Cut into squares