Preheat oven to 180 degrees C. Grease a 20cm square (or round) cake tin.
In a bowl over simmering water, melt the butter and chocolate over a low heat, stirring as necessary. In a separate bowl , beat the egg whites with 1/2 teaspoon caster sugar and pinch of salt until stiff peaks form, as for meringue. In larger bowl beat the egg yolks with the sugar and then slowly fold in the sifted flour. To this mixture add the chocolate and butter; reheated if it has started to go solid.
After the chocolate has been thoroughly mixed in, fold in the egg whites, about a third at a time. When this is well mixed, pour into the cake tin and bake for approx 25 minutes or until a skewer inserted in the cake comes out clean.