In a medium bowl, mix together the flour, sugar and salt. In a separate bowl, whisk the softened butter until it is completely smooth and mayonnaise-like in texture. Using a stiff spatula or spoon, beat the egg whites into the dry ingredients until completely incorporated and smooth. Whisk in the softened butter by thirds, scraping the sides of the bowl as necessary and whisking until the batter is creamy and without any lumps. Transfer the batter to a smaller container, as it will be easier to work with.
I only had four ramekins to shape the tuiles, so I did the rest as flat crackers, which were just as tasty and much easier to make!