In a large bowl, cream together softened butter, sugar, and salt with a hand mixer, until light and fluffy. Add in egg and beat to incorporate.
Sift in flour, cocoa powder, and baking soda using either a flour sifter or a fine mesh sieve. (You don’t want any clumps!) Beat well to combine.
Turn out dough on a clean surface and shape into a flat square or circle. Wrap in plastic and refrigerate 1 hour so it’s firmer to work with.
To prepare cookies:
Preheat oven to 325°F (160°C).
Remove dough from fridge, working with ¼ of the dough at a time, and roll out between sheets of parchment until ¼-inch thick. (Keep remaining dough stored in fridge.)
Use a small round cookie cutter or champagne glass to cut into circles. Either peel outer edges of dough scraps away to separate from cut circles, or use a spatula to help lift the cut dough circles. Transfer to prepared baking sheet lined with parchment and sprayed with cooking spray.
Reshape scraps into dough ball and continue rolling out sections of remaining dough until all cookies are cut.
Bake cookies for 15 minutes.
Allow to cool completely before removing from tray.
Assemble cookies with cream filling for completed Oreo sandwich cookies. (See below.)