Preheat oven to 350 F. Line muffin pan with paper liners.
In a large bowl, sift together the all-purpose flour, cocoa powder, sugar, baking powder, baking soda, and salt then whisk to combine. Set aside.
In a large mixing bowl, whisk the eggs, oil, sour cream, milk and vanilla extract until combined.
Use a rubber spatula to gradually stir the wet ingredients into the dry ingredients. Stir until just combined, being careful not to over mix.
Pour a little less than ¼ cup of batter into each cupcake liner (they should be between ½ to ⅔ full). Bake at 350℉ for 16-18 minutes or until a toothpick comes out clean when inserted in the middle of a cupcake.
Remove cupcakes from the pan. Set them on a wire rack to cool completely.
Filling the Cupcakes
Once cooled, use a cupcake corer (or use the wide end of a large piping tip or a small paring knife) to carve out a hole on top and to remove the center of each cupcake. Be sure not to cut all the way to the bottom if using a knife. Save the centers or snacking!
Spoon the Nutella into a piping bag without a tip or in a zip lock sandwich bag. Snip off one of the corners and pipe about 2 teaspoons worth of Nutella in each cupcake.
Nutella Frosting
In a large mixing bowl or the bowl of an electric mixer with the paddle attachment, beat the butter on medium high speed for one minute. Add the Nutella and beat the Nutella and butter for a minute.
With the mixer on low speed, add the powdered sugar, vanilla, heavy cream or milk and a pinch of salt. Increase the speed to medium high and beat for two more minutes or until creamy, light and fluffy.
Notes
Doneness: Insert a toothpick into the center of a cupcake. If it comes out clean or with a few crumbs attached, the cupcakes are done. Powdered sugar: Be sure to sift the powdered sugar to remove lumps.Sour cream: Use full fat plain yogurt as a substitute for sour cream. Vegetable oil: Canola oil or avocado oil work in place of vegetable oil.Frosting: Allow cupcakes to cool completely before stuffing and frosting. Use a cupcake corer or the wide end of a piping tip to remove the center of the cupcakes before filling. I recommend a Wilton 1M or 6B piping tip or an Ateco 828 to frost these cupcakes. You can also use an offset spatula or a cookie scoop or ice cream scoop to frost the cupcakes.Decorating: Top with mini or regular chocolate chips, chopped hazelnuts or chocolate shavings or sprinkles!Storage: Store plain cupcakes in an airtight container at room temperature for up to 2 days. Store filled and frosted cupcakes in an airtight container at room temperature for up to 8 hours or in the refrigerator for up to 3 days. They taste best when served at room temperature. Let them sit at room temperature for about an hour before serving.