Toast slices of bread. Spread butter thinly, together with dressing. Divide among the slices corned beef, cole slaw and cheese. Arrange on a baking sheet and bake in a preheated 400 degree oven for 5 to 7 minutes, until cheese is lightly brown, about 1 minute. Cut Reuben in half on the diagonal. Serve immediately, accompanied with dill or half-sour pickle optional.