Preheat oven to 400 degrees. Add liners to a muffin tin and set aside.
In a large bowl, vigorously whisk eggs and brown sugar until light and thickened.
Slowly stream oil into the bowl, whisking continuously until combined.
Add Greek yogurt, vanilla and buttermilk. Whisk until incorporated.
In a separate bowl sift together flour, baking powder, Dutch-process cocoa powder and salt.
Toss in chocolate chips and stir to coat with flour mixture.
Gradually add dry ingredients to wet. Stir until just combined to avoid overmixing. (Do not break up dry lumps in the batter.)
Distribute batter evenly among muffin liners filling each 3/4 of the way.
Swirl 2 teaspoons of Nutella on top of each muffin.
Bake for 5 minutes at 400 degrees then, keeping the oven door closed, lower the temperature to 350 and bake for an additional 10 to 12 minutes. Muffins are done when a toothpick inserted in the center of a muffin comes out with a few moist crumbs.
Transfer muffin tin to a wire rack to cool for 5 minutes before removing muffins from pan to cool completely.