Preheat the oven to 390°F/180°C.
In a standing 5-quart mixer with a paddle attachment, beat the butter and sugar on medium-high speed for 5 minutes. The butter should look light and fluffy. Make sure to scrape down the sides of the bowl with a rubber spatula.
One by one, add the egg whites and then the vanilla to the creamed butter. Add a white, beat for 30 seconds, then scrape down the sides of the bowl with a rubber spatula and repeat.
Sift the flour and salt a few times so there are no lumps. Slowly mix in the flour into the butter mixture.
Using a pastry bag with a #2 tip, fill the bag with the cookie batter and pipe on to a sheet tray that is lightly buttered or onto a silpat. Be sure to leave 2 inches between the cookies as you pipe, as they do spread a little.
Bake the cookies until a nice golden brown appears on the edges, about 12–15 minutes. Remove from the oven and allow them to cool completely.