1 cup sugar
1 egg yolk
1/2 stick (1/4c) butter cut into 4 pieces
Set up an ice bath by putting a medium bowl in a larger bowl filled with ice water. Set fine mesh sieve strainer over the top of the smaller bowl and set aside.
Finely grate enough zest from lemons to measure 1 Tbs and squeeze enough juice to measure 1/2 cup.
Combine zest and sugar in bowl and blend thoroughly with fingers.
Whisk together sugar and eggs in metal bowl. Set bowl over sauce pan of simmering water and whisk until sugar has dissolved, about 1 min. Add lemon juice and continue whisking until thick and smooth and an instant thermometer reads 160 degrees. About 5-7 min.
Add butter and whisk until well combined. Remove from heat and strain into prepared bowl. Let mixture cool completely then transfer to air tight container and refrigerate over night. Will keep for about 1 week. Covered and chilled.