In a medium saucepan, reduce the grapefruit juice by half to 1/2 c over medium-high heat. Allow to cool slightly.
Add eggs, egg yolks and sugar and cook over low heat until mixture begins to thicken and coats the back of a spoon, 10-12 minutes. Remove from heat and stir in butter one tablespoon at a time until smooth. Stir in grapefruit zest.
Pour in bowl or jar and cover top with plastic wrap to prevent a skin from forming. Keeps in the fridge for a week, in the freezer for a month.