Preheat the oven to 350°F and line two cookie sheets with parchment paper. Set aside.
In a medium bowl, sift the flour and baking soda or whisk well to combine.
In a separate bowl, using a hand mixer cream the softened butter and brown sugar until fluffy. Add the egg, salt, vanilla, cinnamon, and cocoa powder. Mix well to combine until fully incorporated.
Combine the flour mixture into the butter, then mix until just combined. Add in the rolled oats and mix until homogenous, without overbeating.
Use a cookie or ice cream scoop to form 20 individual cookies. Place them onto the lined cookie sheets, at least 2-3 inches apart.
Bake for 8 minutes or until golden brown. For extra flat cookies, press them down with a spatula as soon as they come out of the oven. This also helps make the top appear cracked. Afterwards, carefully transfer them to a cooling rack.