Measure and prepare all ingredients. Keep the butter refrigerated until use. Zest and juice the lemons, totaling about 1/4 cup zest and 1/3 cup juice.

In a double boiler or heatproof bowl over simmering water, whisk together egg yolks and sugar until well combined.

Add lemon juice and zest to the egg mixture, whisking continuously, and cook over medium heat for about 8-9 minutes until thickened.

Remove from heat. Add cold butter one tablespoon at a time, whisking until smooth each time. Pour into a container, cover, and refrigerate until cooled and thickened, about 2 hours.