Before starting this Langues de Chat Biscuits recipe, make sure you have organised all the necessary ingredients. In a mixing bowl, place the soft butter and castor sugar. Beat vigorously with a whisk until pale and fluffy. Add the sifted flour, egg whites... ...and a few drops of vanilla extract. Combine with a spatula until homogeneous. Transfer the preparation into a piping bag fitted with a polycarbonate plain nozzle (diameter 10mm). ...