Preheat the oven to 425°F. Spray 2 12-cup muffin tins with baking spray or add liners, set aside.
In a medium-sized bowl, stir together the flour, cocoa powder, baking powder, baking soda, and salt, set aside.
In a large bowl, whisk together the eggs and sugar until smooth. Add the milk, oil, sour cream, and vanilla, carefully whisk until smooth.
Add the dry ingredients to the wet and stir to combine until there are no dry patches.
Add 1 and ½ tablespoons of the batter to the bottom of 22 sections of the greased/lined muffin tins. I like to use a 1 & ½ tablespoon-sized cookie scoop for this. Add 1 heaping teaspoon of Nutella to the center of each muffin. Add another 1 & ½ tablespoon of the batter on top. Make sure the batter on top completely covers the Nutella.
Bake for 5 minutes at 425°F. Lower the oven to 350°F, leaving the muffins in the oven, and bake for an additional 10-13 minutes. The muffins will puff up, slightly crack on top and will appear dry when ready. You can insert a toothpick into the center of the muffin halfway between the center and edge, you may get some nutella on the toothpick but the muffin batter itself should come out clean.
Let the muffins cool in the muffin tin for 5 minutes. Take them out and let them cool completely on a wire rack.