In a medium bowl, whisk together the flour, cocoa powder, baking soda and salt.
In the bowl of a stand mixer fitted with the paddle attachment (you can also use a large bowl and hand mixer), combine the butter, brown sugar and granulated sugar. Beat on medium speed until light and fluffy, about 2 minutes.
Add the eggs and vanilla and beat until well combined, scraping down the sides of the bowl as needed.
With the mixer on low speed, slowly add the dry ingredients and mix until just combined. Mix in the oats and 1 cup of the chocolate chips on low speed (reserve remaining 1/2 cup of the chocolate chips for pressing into the tops of the cookies after baking, if desired).
Chill the dough for at least 1 hour and up to 24 hours.
When you are ready to bake the cookies, preheat the oven to 350 degrees F. Using a cookie scoop or spoon, drop heaping tablespoons of dough onto parchment-lined baking sheets. Press the tops of the cookies down just a little bit, as the cookies won't flatten much on their own.
Bake the cookies for 8-10 minutes, until tops are almost set. Be careful to not over-bake the cookies. If you are baking more than one pan of cookies at a time, rotate the pans halfway through the baking time.
As soon as the cookied come out of the oven, press a few chocolate chips into the tops of cookies. Let cookies cool on the baking sheets for 5 minutes and then transfer to a wire rack to finish cooling.