Preheat oven to 400°F. Vigorously whisk butter and sugar in a large bowl until light and fluffy. Whisk in eggs, one at a time. Whisk in vanilla and sour cream.
Mix flour, cocoa powder, baking powder, espresso powder, and salt in a large bowl until combined.
Make a well in the center; pour in wet ingredients and fold with a wooden spoon or rubber spatula until combined. Fold in chocolate chips
Fill muffin liners 2/3-full (I use a 2 oz cooke/ice cream scoop and it's just the right amount). Heat nutella in the microwave in 15-second intervals until smooth and slightly runny, stirring between each interval if necessary. Use a spoon to dollop nutella on tops of batter in a circular motion.
Bake 5 minutes, lower temperature to 350°F, and bake additional 12 to 15 minutes.
Cool muffins in pan 5 minutes before transferring to a wire rack to cool completely. Store in an airtight container in the refrigerator 3 to 5 days.