Melt 1 (12 ounce) bag of chocolate chips and 2 tablespoons of butter over low heat. Remove from heat, add 1 can of sweetened condensed milk. Stir until smooth. Add 1 (10 1/2 ounce) bag of miniature marshmallows and 1 cup of nuts. Stir until coated. Spread into 13 x 9 pan that has been sprayed with no-stick spray. Cool, cut into bars.