Brace yourselves for Double Chocolate Chip Muffins. Let’s be real - it’s essentially cake in a muffin tin, but who cares. Double chocolate all the time.
Preheat oven to 375 degrees. Line a standard 12-cup muffin tin or a jumbo 6-cup muffin tin with paper liners.
In a bowl, combine flour, ¼ cup cocoa powder, baking powder, baking soda and salt; set aside. In a separate bowl, whisk remaining cocoa powder and water together until smooth.
In the bowl of a stand mixer fit with a paddle attachment, beat sugar, vegetable oil, eggs and yolk, and vanilla extract on medium until combined, about 30 seconds. Add cocoa powder mixture and beat until smooth. In three additions, add flour mixture, alternating with sour cream, until combined. Scrape down the sides of the bowl and add 2 cups chocolate chips. Beat until just incorporated. Divide batter evenly between prepared muffin tin, filling paper liners all the way up. Sprinkle with remaining chocolate chips.
Bake in the center of the oven until muffins have risen above the pan, and a toothpick inserted into the center comes out clean, about 25 minutes for standard muffins, and 35 for jumbo.
Transfer to wire rack to cool slightly. Enjoy warm or at room temperature.
Notes
The trick to bakery-worthy muffins is to always press the chocolate chips into the top of each muffin before baking. You can stir them all in if you are in a rush and not looking for fancy presentation.