Preheat the oven to 350°F (175°C) and line a 12-cup muffin tin with paper liners.
In a large bowl, whisk together flour, cocoa powder, sugar, baking soda, and salt.
In a separate bowl, mix warm water, vegetable oil, vinegar, and vanilla extract.
Stir wet ingredients into the dry ingredients until just combined to form the batter.
Divide the batter evenly among liners and swirl in warmed Nutella using a toothpick.
Bake for 18–22 minutes or until a toothpick comes out clean.
Let the cupcakes cool completely on a wire rack.
Beat butter, Nutella, powdered sugar, cream, and vanilla until smooth. Frost cooled cupcakes.