Grate the zest finely from the lemons, then squeeze and strain 6 ounces of juice. Combine the juice, zest, sugar, and butter in a saucepan and bring to a boil over medium heat. Whisk the yolks in a bowl until liquid. Strain out the zest and beat 1/4 of the boiling liquid into the yolks and return the remaining liquid to a boil over low heat. Whisk the yolk mixture into the boiling liquid and continue whisking until it thickens slightly, about 2 minutes. Do not allow the lemon curd to boil or it will scramble, although it may come to a slight simmer at the edge of the pan. Pour the lemon curd into a clean container, press plastic wrap against the surface and chill.