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.
Slivered nuts toasted are nice in the meringue if you like. For this amount of meringue, about 2/3 cup/60 g is good.