Preheat oven to 325 degrees F.
Slice bread into 1/2-inch thick slices, butter each slice, sprinkle with Parmesan and place on a baking sheet. Bake until golden brown.
Whisk cream, dry mustard, Worcestershire sauce, and cayenne pepper together in a saucepan. Heat over medium-low heat, stirring constantly and gradually adding Cheddar. Heat until flavors are integrated, cheese is melted, and mixture is thickened. Serve as a fondue in which to dip bread crisps.