Preheat your oven to 375°F / 190°C. Generously grease two baking sheets with butter, or line them with parchment paper or silicone mats.

In a medium bowl, whisk the egg whites until they are light and foamy.

Sift in the confectioners' sugar, cocoa powder, and flour.

Mix gently until just combined, scraping down the sides of the bowl to ensure no dry spots remain.

Pour in the melted butter and stir until the mixture is smooth and fully incorporated. The consistency should be like a thick, spreadable batter. If it seems too stiff, add a bit more melted butter, one teaspoon at a time.

Drop teaspoonfuls of batter onto the prepared baking sheets, leaving several inches of space between each one.

Using the back of a spoon or a small offset spatula, spread each dollop into a very thin 2- to 3-inch circle.

Bake for 8 to 10 minutes, or until the edges are slightly darkened and the centers are firm to the touch.

Work quickly while the cookies are still hot. Use a thin metal spatula to lift them from the baking sheet. To create the classic curved shape, immediately drape them over a rolling pin, dowel, or the side of a glass. To cool them flat, transfer them to a wire rack. They will become crisp as they cool.

Bake in small batches of 3 or 4 cookies at a time. They cool and harden within seconds of leaving the oven, so you need to work very quickly to shape them.