1) Wash grapefruit and pat dry. Grate zest finely into a bowl. Squeeze juice, add to zest and leave to infuse for about 30 minutes; after which strain and discard zest.
2) Pour juice into a clean bowl and add sugar and butter. Heat over a pan of simmering water. When the butter is melted, beat in eggs. Stir continuously over heat until the mixture thickens and coats the back of a wooden spoon.
3) Pour the thickened mix into prepared jars, seal and label.
