Prepare all ingredients: zest the lemons, juice the lemons, and measure all ingredients. Keep the butter cold until ready to use.

In a double boiler or heatproof bowl over simmering water, whisk together egg yolks and sugar until smooth and slightly thickened.

Gradually add lemon juice and lemon zest to the egg mixture, whisking constantly. Continue over heat for about 8-9 minutes until mixture thickens slightly.

Remove from heat. Add butter piece by piece, whisking until smooth and melted.

Pour the lemon curd into a clean bowl, cover with plastic wrap directly on the surface, and refrigerate for at least 2 hours until chilled and thickened.