Heat your oven to 375°F (190°C). Line a cookie sheet with parchment paper and have another sheet of parchment paper ready. 

Using an electric mixer, blend together the sugar, butter and salt in a large bowl. Add the egg whites one at a time to the bowl, blending between additions. Blend in the vanilla extract. Using a spoon, stir in the flour a bit at a time until all is incorporated. 

Fill a pastry bag without a tip (or cut a ½”/1cm opening if using a disposable pastry bag) with the batter mixture. Pipe flattened circles 1 ½”/3.5cm in diameter onto the parchment paper lined cookie sheet. Make sure the circles are 2”/5cm apart. Place the cookie sheet with cookies in the oven and bake for 10-12 minutes, until the edges of the cookies appear golden brown. While the cookies bake, continue to pipe out more cookies on the remaining sheet of parchment paper. 

Once the first batch of cookies is baked, simply slip the parchment with the baked cookies off of the cookie sheet. Slide the parchment with the unbaked cookies on. Continue to bake until all of the batter has been piped out and baked. 

Cool the baked cookies on a wire cooling rack. Store in an airtight container.