Cream the room-temperature butter with the powdered sugar and vanilla. Add half of the egg whites and mix. Then add half of the flour and mix again. Next, add the remaining egg whites and flour. Beat only until all the ingredients are fully combined into a smooth batter. As soon as the mixture comes together (photo 1), stop the mixer. Transfer the batter to a plastic bag (photo 2) and refrigerate for 15 minutes.
Cut a 1 cm opening in the corner of the bag, or use a piping bag fitted with a plain 1 cm nozzle. Line a baking tray with parchment paper, then generously grease the parchment with butter and dust it with flour. Otherwise, the cookies will stick. Pipe the batter into strips, leaving about 3 cm (1¼ inches) between each one (photo 3), as the cookies spread considerably while baking.
Bake in a preheated oven at 200°C (390°F) for 10–12 minutes, or until the Cat's Tongue Cookies reach the desired golden color. They should dry out well in the oven; otherwise they will remain soft, whereas they should be dry and crisp. Remove the cookies from the oven, let them cool for just 1–2 minutes, then immediately lift them off the parchment paper with a spatula and transfer them to a wire rack to cool completely.
Notes
Be sure to remove the cookies from the baking tray while they are still warm. If they cool completely on the parchment paper, they will stick and be difficult to remove.