Preheat oven to 350 degrees F. Line muffin tin with paper baking cups or grease with cooking spray.
In a large bowl, beat together sugar, peanut butter, sour cream, and eggs. Mix until well combined.
On low speed mix in the flour, cocoa, water, and baking soda until no more white streaks remain.
With a spoon, stir in the chocolate chips. Spoon into the muffins cups.
Bake for 15-20 minutes, or until an inserted toothpick comes out clean.
Cool for a few minutes in the pan and then transfer to a wire rack to finish cooling.
When the muffins are cool, sprinkle with powdered sugar if desired.