Melt 1 tablespoon of the butter in a 12-inch cast iron skillet or broiler-safe frying pan over medium-high heat. Swirl the pan to coat the pan evenly with butter, then add 2 slices of the bread. Fry (that's right — I said fry) until golden-brown on the bottom, 2 to 3 minutes. Transfer the slices to a wire cooling rack. Repeat with the remaining butter and bread slices.