Preheat canner water to 180°F; do not exceed 180°F before loading jars.
Mix sugar and optional zest and stand 30 minutes. Whisk yolks and eggs, slowly whisk in sugar mixture, bottled lemon juice, and butter.
Cook over gently boiling water, stirring continuously, to 170°F. Remove from heat, stir about 5 minutes until thickened, then strain.
Fill hot half-pint jars with 1/2-inch headspace. Process 15/20/25 minutes at 0–1,000/1,001–6,000/above 6,000 ft.