Whisk eggs, yolk, and sugar together in a heatproof bowl. Set a strainer in a separate bowl. Set both aside.

In a pan over medium heat, melt the butter with the lemon juice, heavy cream, and lemon zest. Don’t worry if the cream appears to curdle slightly. Heat until nearly boiling, when small bubbles appear on the edges of the pan.

Very slowly pour the hot liquid into bowl with the eggs and sugar, whisking continuously.

Now pour all of the contents of the heatproof bowl back into the pan and bring to medium heat.

Stirring continuously with a wooden spoon and frequently scraping the bottom and sides of the pan, heat the mixture for about 5 minutes. You’re ready to take it off the heat when the mixture quickly thickens to the point where it coats the spoon without dripping off. Don’t heat too fast, and keep stirring so that the eggs don’t overcook and scramble.

As soon as the mixture has thickened, pour into your remaining bowl through the strainer. Dispose of any solids that stick in the strainer.

Let cool in the refrigerator for 2 hours.