Make 1 2/3 cups
4 large egg yolks, plus one whole egg, lightly beaten
1 c. sugar (for the triple batch I used 2.5 +/- cups)
1/2 c. freshly squeezed lemon juice, strained (3 lemons)
1/2 c. (1 stick) unsalted butter, cut into pieces
1 tbsp. grated lemon zest
1. Strain the egg yolks and the whole egg through a sieve into a medium-heavy non-reactive saucepan. Add the sugar and lemon juice; whisk to combine.
2. Cook over low hear, stirring constantly with a wooden spoon until the mixture thickens and coats the back of the spoon, 10-12 minutes (I generally make a triple batch which take about 20.) Remove from heat; transfer to a heat-proof bowl. Stir in the butter one piece at a time, until fully incorporated. Stir in lemon zest. If not using immediately for the tart, cover the surface of the lemon curd with plastic wrap to keep a skin from forming. Refrigerate in an airtight plastic container up to 3 days.