Preheat the oven to 350F (180C). Line cookie sheets with parchment paper or baking mats.
Whisk together the flour, cocoa, baking powder, baking soda and salt. Set aside.
In a large bowl, beat together the butter, brown sugar and granulated sugar until combined
Mix in the egg and vanilla extract.
Add the dry ingredients into the butter mixture. Starting with the mixer on a low speed, beat the dry ingredients into the butter mixture.
Turn off the mixer. Add in the chocolate chips and chopped Oreos. Stir them into the batter with a rubber spatula or wooden spoon.
Form the dough into balls. For large cookies, use about 3 tablespoons of dough per cookie (the size of a large cookie scoop, or 2 scoops of a medium cookie scoop). For smaller cookies, make dough balls with about 1 to 1.5 tablespoons of dough. If the dough feels very sticky, place in the fridge for about 30 minutes.
Place dough balls 2-3 inches apart on the lined cookie sheets. Bake 1 sheet at a time in the middle rack of the oven. Large cookies will need about 12-14 minutes and smaller cookies will need about 8-11 minutes.
Remove from the oven and optionally place a few extra chocolate chips and Oreo pieces on top of each cookie. Cool on the cookie sheet for at least 10 minutes, then transfer to a wire rack to continue cooling.