Place a rack in the middle of the oven and heat to 375ºF.
Lube a muffin tin with nonstick cooking spray. I do this inside a plastic grocery bag to keep the spray from going everywhere.
Sift the flour, cocoa powder, baking powder, baking soda, and salt together in a large bowl. Air is an important ingredient here, so it’s not enough to simply stir the powders together. If you don’t have a sifter, pulse the mixture in your food processor.
Stir in the chocolate chips.
In a separate bowl, whisk together the sugar, buttermilk, unsalted butter, eggs, and vanilla extract.
Add the wet ingredients to the dry and stir until just combined using a large rubber or silicone spatula. Resist the urgeto mix it smooth — lumps are good.
Using an ice cream disher or spoon, drop the batter into the prepared tin. The cups should be full.
Park on oven rack and and boost the temperature to 400ºF.
Bake until the internal temperature hits 210ºF on a digital instant-read thermometer or a toothpick inserted in the bottom of a muffin comes out clean, 18 to 20 minutes.
Remove from the oven and immediately turn the muffins up on their sides so that steam can escape the pan. Skip this step and you'll have gummy bottoms.
Serve immediately or store in an airtight container.