Preheat oven to 450 degrees, with rack in upper third. Make deep, even cuts into bread, about 3/4 inch apart, without cutting through bottom. Stir together butter, Parmesan, lemon zest, and chives. Season with salt and pepper. Spread mixture evenly between cuts, wrap loaf tightly in parchment-lined foil, and bake until butter is melted and crust is crisp, about 15 minutes. Unwrap and let cool slightly before serving.