In a pan, melt together butter and chocolate over a low heat, stirring as necessary.
In a bowl beat the egg whites with 1/2 tsp caster sugar and pinch of salt until stiff peaks form, as for meringue.
In another bowl beat the egg yolks with the sugar and then slowly fold in the flour. To this mixture add the chocolate and butter (if this has started to go solid, re-heat a little until liquid). After the chocolate has been thouroughly mixed in, fold in the egg whites, about a third at a time. When this is well mixed, pour into a lightly buttered 20 or 23cm square tin and bake for approx 25 minutes in a 180 C oven. To test for doneness, place a metal skewer in the centre. If it comes out clean, your cake is ready.