To a large mixing bowl, add your butter, brown and white sugar and beat them together with a mixer for a minute, till creamy. Add your vanilla and egg and briefly blend in.
Place a fine mesh strainer over the same mixing bowl. To the strainer, add your flour, cocoa powder, baking soda and salt. Gently shake the strainer back and forth to strain the dry ingredients through, while keeping any cocoa powder clumps out. Remove the strainer and discard any cocoa powder clumps.
Stir your wet and dry ingredients together till you have a smooth cookie dough batter. Add your oats and chocolate chips and fold them into your dough.
Let your cookie dough rest and preheat your oven to 350. Line two pans with parchment paper or spray with non-stick spray.
Using a small cookie scoop, scoop the dough onto your pans evenly spaced apart (with no more than 12 cookies per pan). Gently flatten the tops of your cookies, as these cookies don't spread much. Bake in the oven for 9-11 minutes, or till the edges of the cookies are set but the middle looks a touch underdone still.
Let cool, then devour!
Stored in an airtight container these cookies taste best within the first 5 days. They can also be frozen for a few months.