Put the butter and caster sugar into a bowl and whisk using an electric whisk until pale and fluffy. Gradually whisk in the eggs one at a time. Fold in the flour, grapefruit zest, vanilla and milk ...
Grapefruit make excellent marmalade. Their pithy skin is also good candied, if blanched several times in fresh water, then simmered in sugar syrup. Otherwise, cut away the thick bitter pith and ...