Preheat oven to 350-degreed Fahrenheit.
In a large bowl, cream together the butter and sugars until light and fluffy (approximately 3-4 minutes).
Add the vanilla and eggs (one at a time, blending after each addition) until incorporated and scraping down the sides of the bowl as needed.
In a large bowl, whisk together the flour, baking soda, salt, and cocoa powder.
Add the flour (in increments) to the butter/sugar mixture, mixing lightly after each addition.
Blend in the oatmeal until the mixture just comes together into a soft dough, scraping down the sides of the bowl as needed.
Use a medium size cookie scoop (or a heaping tablespoon), to drop cookie dough onto parchment lined baking sheets leaving approximately 2" between each cookie.
Bake the cookies for 15-17 minutes in a preheated oven (rotating the sheets halfway through the baking process). *Note - the cookies will puff in the oven and deflate as they cool. They should be only slightly soft to the touch when hot as they will firm up as they cool.
Cool the cookies on a wire rack.