Preheat oven to 350ºF and line 2 large baking sheets with parchment paper.
Add egg whites to a medium bowl and whisk until lightly frothy. Add the sugar and whisk vigorously until very frothy and white.
Add the flour and stir until a smooth batter forms. Then add the melted butter and vanilla extract and stir until combined. If not baking immediately, transfer to the fridge.
Spoon around 1-2 teaspoons of batter onto the lined baking sheet and spread into a very thing circle (around 3 inches in diameter). Leave some room in between each circle (around 6 cookies per sheet). If using toppings, sprinkle them on now.
Bake in the preheated oven for 5-8 minutes, or until just lightly golden around the edges. If you prefer a chewier cookie, bake for 5-6 minutes. For a crispier tuile, bake for 7-8 minutes. While the first batch bakes, start spreading out the batter for the second batch. It may take 4-5 batches to use all of the batter.
Once done baking, remove from the oven. Work quickly to shape the tuiles if desired. They should peel off of the parchment paper. Drape them over a rolling pin or roll around a small wooden dowel or tube. If they cool too quickly, you can put them back in the oven for 30 seconds - 1 minute and they will become flexible again.
Allow the tuiles to cool completely before enjoying.