Thoroughly grease a 9-inch-by-13-inch baking dish with softened butter.
Add the 4 tablespoons butter to a medium pot over medium heat. When the butter is melted, stir in the white mini marshmallows and heat until they're totally melted. Stir in the salt. Remove from the heat and pour in the cereal, folding gently until it is all combined. Then gently fold in the rainbow mini marshmallows. Press into the prepared baking dish. Let cool completely.
Melt the bark according to the package directions. Cut the treats into 12 pieces. Dip half of each piece into the white chocolate and place onto wax paper. Let set and serve.