Preheat the oven to 350F and line the baking sheets with parchment paper.
With a mixer, mix together butter and sugar in a large bowl until smooth. Add eggs and vanilla and mix until smooth.
Add flour, baking powder, salt and cocoa powder and mix until combined. Add the chocolate chips and stir until they are evenly incorporated into the cookie dough.
Take 1 heaping tablespoon of the dough and roll with your hands into a ball. Put the ball on a parchment-paper-lined baking sheet. Repeat with remaining dough, placing the balls 2 inches apart, as the cookies will spread.
Put in the oven and bake for 10 minutes. The cookies will look undercooked, but they will set as they cool. If you want them to be soft and chewy, do not overbake!
Notes
If your cookie dough is too sticky to roll into balls, just drop it by a heaping tablespoon onto the baking sheet.