Preheat your oven to 350 F. Line two baking sheets with parchment paper. Set aside.
In a medium size mixing bowl, whisk together the flour, cocoa powder, baking soda and salt. Set aside.
In a separate mixing bowl, beat together the butter and sugars until smooth. Stir in the egg until well mixed.
Gradually add the dry ingredients and stir on low speed until mixed.
Using a cookie scoop or spoon, create 1.2 oz. balls of cookie dough. Roll each into a smooth ball, placing them about 2-3 inches apart on the cookie sheet (for a standard cookie sheet, you should be able to fit 12 cookies). Place a small square of parchment on the first ball of cookie dough, then use the bottom of a measuring cup or drinking cup (as long as it's flat and smooth) and gently press the cookie dough down (it should be about 1/3-1/2 inch thick). Repeat for all of the cookies.
Bake the cookies for 6 to 7 minutes. Allow to cool completely before making the cookie sandwiches.