In the bowl of a stand mixer fitted with a paddle attachment or a large bowl, beat together the butter, sugar, and vanilla sugar until pale and fluffy.
Beat in the egg whites, one at a time.
Mix in the flour and salt just until combined.
Transfer to a pastry bag with a 1/2 inch (1.25 centimeter) round tip and place in the refrigerator for 20-30 minutes.
Preheat oven to 375˚F (190˚C). Line two baking sheets with parchment or lightly grease.
Pipe 3 inch (7.5 centimeter) long strips on prepared baking sheets, leaving about 2 inches (5 centimeters) between each strip.
Bake in the preheated oven until golden around the edges, 6-8 minutes, rotating the pans halfway through the cooking time if needed.
Allow to cool to room temperature on the baking sheets.
If desired, dip the cooled cookies in the melted chocolate and allow to set before serving.