Melt the butter in a small saucepan and set aside to cool. Mix together the sugar, powdered walnuts, and flour. Stir in the egg whites and Cognac. Finally, whisk in the butter.
Heat the oven to 450ºF/230ºC. Fill the financier or mini muffin tins three-quarters full only, and bake five minutes. Reduce the oven temperature to 400ºF/200ºC and continue baking until they are golden on top, and a toothpick inserted in the centre comes out clean, about 10 minutes. If you’re working in batches, don’t forget to turn the oven back up to starting temperature before putting them in.