Whisk together the sugar, three eggs, and egg yolk in a medium sized microwave proof bowl. Then whisk in the grapefruit juice and zest. Finally whisk in the butter.
Microwave at 50% power for about a minute. Stir, and put back in the microwave for another minute. Stir again and continue microwaving for one minute intervals until when you dip a spoon in, it forms a thick layer on the back of the spoon.
Strain it through a mesh metal strainer to remove the zest and refrigerate to cool. It will thicken up more as it cools. Store in the refrigerator for up to two weeks.