Directions

1. Put water in a double boiler, or put a few inches of water in a pan and put a metal bowl on top, making sure the water doesn't touch the bowl.
2. Bring the water to gentle boil.
3. Meanwhile, zest the lemons.
4. In a food processor, pulse the sugar and zest together thoroughly.
5. Juice the lemons. Make sure you have just about a half cup of juice.
6. Add the eggs yolks and sugar/zest mixture to the top part of the double boiler/the metal bowl.
7. Whisk together.
8. Gradually add the lemon juice while whisking.
9. Keep whisking until the mixture thickens. This might take a bit; be patient.
10. Remove from heat.
11. Cube the butter and add to mixture. Stir until melted.