Giant saffron buns were the traditional treat at Cornish Sunday-school 'tea treats', and bakers still call the large ones tea treat buns.
A rich, yeasted dough studded with currants and sultanas is tinted golden with saffron, then baked into loaves or individual buns. Cornish bakeries sell them sliced and buttered as an everyday treat, not just for special occasions.
Why it's made this way: Saffron likely arrived via Cornwall's ancient trading connections abroad, and the county held onto the habit of cooking with it long after the spice fell out of everyday use elsewhere in Britain.