In a large pot over medium-low heat, combine the butter, peanut butter, and butterscotch chips. Stir frequently until the mixture is completely melted.
1/2 cup (113g) unsalted butter, 1 cup (270g) creamy peanut butter, 11 oz butterscotch chips
Remove from the heat and stir in coconut, walnuts, and marshmallows. Pour the mixture into an ungreased 9x13 inch pan and spread to even it out.
1 cup (85g) sweetened flaked coconut, 1 cup chopped walnuts, 10 oz mini marshmallows
Put the pan in the refrigerate to set, then cut into bars.