In a medium saucepan, whisk eggs, egg yolks and sugar together until smooth. Whisk the corn starch together with a bit of the grapefruit juice to create a slurry.
Whisk the grapefruit juice and slurry into the egg mixture.
Add the food coloring if using.
Continue to whisk over medium heat until thick, 10-15 minutes. It should be the consistency of pudding. Make sure the whisk hits the bottom of the pot to keep the bottom from burning. This is less likely to happen if you use a heavy bottom saucepan.
Remove from the heat and slowly whisk in pieces of the butter.
Move the curd to a bowl and allow to chill in the refrigerator. It will continue to thicken as it cools.
Store extra curd in the refrigerator with plastic wrap pushed directly against the curd to prevent a skin from forming.
To make the ice cream:
Whisk the curd and half and half together until well combined.
Sift in one tablespoon of confectioners sugar at a time while whisking.
Prepare according to your ice cream maker manufacturer's instructions.
I prefer to make this recipe over 2 days since the curd only takes 15 minutes to prepare but needs time to chill thoroughly. If you prepare the curd the night before, it will only take 25 minutes the next day to finish the ice cream.