Heat the oven to 325°F/160°C.
Butter a 3-cup/750 ml soufflé dish, or 6 to 8 1/2-cup/125 ml ramekins, and sugar the insides, just as you would flour a cake tin.
Beat the whites, cream of tartar, and salt to soft peaks.
Scatter over one tablespoon of the sugar and beat until stiff and glossy.
Fold in 3 tablespoons more sugar.
Pour the mixture into the mould and bake until a skewer inserted in the centre comes out clean, about 25 minutes for a single dish, or 20 minutes for ramekins.
About 5 minutes before the “islands” come out of the oven, melt the remaining 1/4 cup/50 g sugar in a saucepan with a splash of water.
Remove from the heat when it turns a dark, amber caramel.
Take the “island” from the oven and invert into a large serving dish with a rim.
Pour the very cold custard all around, and drizzle over the dark, mahogany caramel.