Preheat the oven to 355F (180C) and line two baking sheets with parchment paper.
In a mixing bowl, add the flour, salt, and baking soda. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment (or you can use a bowl and a hand mixer), add the butter, white and brown sugar, and vanilla extract. Beat on medium-high for 2 minutes or until it resembles a homogenous paste.
Then, add the egg and beat until emulsified.
Add the dry ingredients and mix until fully incorporated.
Scoop 20 even dough balls onto the prepared baking sheets. Bake one at a time for 10-12 minutes, or until the cookies have spread and the edges turned a golden color.
Remove from the oven and drop onto a flat surface to deflate the cookies. Let them rest on the sheet for a couple of minutes. Then, transfer onto a cooling rack to cool down completely.
Bake the second batch. If you have only one baking sheet, the dough can wait in the fridge while the first batch is baking. You should place the cookie dough balls on a completely cooled-down baking sheet.