In a medium bowl, beat the butter, oil and sugar with an electric mixer until light and fluffy. Add the eggs and vanilla and beat until smooth.
The batter is thick – it needs to be about the same consistency as the Nutella so that you can swirl the two together easily.
In a small bowl, stir together the flour, baking powder and salt; add to the butter mixture and beat on low speed just until combined. The batter will be thick. It needs to be in order to swirl in the Nutella!
Divide the batter between 12 paper-lined muffin cups. Drop a spoonful of Nutella on top of each, and swirl through the batter with the tip of a bamboo skewer, knife or other pointy object. Bake for 25-30 minutes, or until springy to the touch. Tilt them in the pan to help them cool. Makes a dozen cupcakes.