6 egg yolks
1 cup of sugar
4 lemons, zested and juiced
1/2 cup unsalted butter, cut into slices and chilled
Place a saucepan on stove and fill with 2 inches of water. bring water to a simmer.
In a metal bowl, that is bigger than the pot with water, combine egg yolks and sugar and whisk until smooth. If the lemon juice doesn't reach 1/3 cup, add water. Add zest and juice to egg mixture and whisk until smooth. Place bowl on top of pot with water and bring temperature down. Whisk until thickened. It should take about 10 minutes. Remove from heat and add butter, a slice at a time, until all melted. Place in a clean jar and cover with plastic film immediately right on top of the curd. Refrigerate for 2-3 weeks.