Preheat oven to 350°F and prepare cupcake pans with 12-15 liners.

Add the flour, sugar, cocoa powder, baking soda, and salt to a large bowl and whisk together. Set this bowl aside. Combine the egg, buttermilk, vegetable oil, and vanilla in another bowl. Add the wet ingredients to the dry ingredients and mix with an electric mixer until well-combined. Add the water to the batter and mix until well combined. The batter will be thin.

Fill the cupcake liners about half-way and bake for 18-23 minutes, or until a toothpick comes out clean. Remove the cupcakes from the oven and allow them to cool for at least 30 minutes before stuffing. To stuff the cupcakes, use a pairing knife to cut a small circle in the tops of each cupcake, and "scoop" out the small section. Reserve the cupcake pieces you remove, because they will go back on top. Add a dollop of Nutella inside each little pocket, and replace the top with the cupcake piece that you removed.

While the cupcakes are cooling, make the frosting. Cream together the butter and cocoa powder. Add in the powdered sugar one cup at a time, mixing well. Add in the vanilla and salt. Finally, add in the milk or cream, one tbsp at a time, until the frosting reaches a smooth consistency. Transfer to a piping bag and pipe onto the cooled cupcakes. Top each cupcake an Oreo and an additional drizzle of Nutella. Enjoy!