Mix the dry ingredients well in a large bowl. Thoroughly stir in the egg whites, then thoroughly mix in the butter and leave for 20 minutes at room temperature.
Preheat the oven to 180°C/350°F/Gas 4. Cover a baking tray with non-stick baking parchment (or grease it well) and dollop 4—6 dessert-spoonfuls of the mix on to the tray, though not too close together as they spread. Using the back of a fork that’s been dipped in cold water, spread each lump into a disc about 10cm (4in) across. Cook in the oven until an even golden colour (about 12 minutes). Remove and, with a metal spatula, take the wafers from the tray and allow to cool on a cake rack. You can also drape them over a rolling pin to form what the French call tuiles (‘roofing tiles’). Store the wafers in an airtight jar.