n a medium sauce pan add the grapefruit juice and ten grams of the sugar. Bring to a boil then lower to a simmer and reduce until you have 180 ml (about ¾ cup) remaining. Pour into a measuring cup or glass and set aside. 

Clean the pan then add the rest of the sugar with the zest. Rub the sugar and zest together with your fingers. Add in the salt and the concentrated juice and lemon juice then turn heat on to medium. Stir until sugar is dissolved and the mixture is hot. 

Separate the eggs if you haven’t done so yet saving the whites in an airtight container in your refrigerator for another use and whisk the egg yolks together in a glass or other non-reactive bowl. Once the juice mixture is hot, slowly add in in a thin stream to the yolks while whisking the yolks not stop to temper them.

Add in about half the juice mixture to the eggs while whisking then pour that mixture back into the pan, scraping up any extra egg yolk with a rubber or silicon spatula. 

Still over medium heat, stir the curd, scraping the spatula along the bottom and sides of the pan, until it has started to thicken.

Remove from the heat and stir in the butter one piece at a time until fully incorporated. 

Transfer to a bowl and cover with plastic wrap, pressing the wrap against the top of the curd then place a lid on the bowl. Chill completely before using. At least 4 hours in the refrigerator. It will thicken more as it cools.