Preheat oven to 400 degrees, with rack in upper third. In a large skillet, melt butter over medium. Add leek; season with salt and cook until tender and slightly caramelized, about 12 minutes. Transfer mixture to a large bowl and add cheddar, Neufchatel cheese, mayonnaise, and stout; mix until smooth. Transfer to a 1-quart baking dish and bake until bubbling and browned on top, 18 to 20 minutes. Serve with mini pumpernickel toasts or crackers.