1Preheat the oven to 350F.
2In a large bowl, beat the butter, oil and sugar with an electric mixer until light and fluffy. Add the eggs and vanilla and beat until smooth.
3In a small bowl, stir together the flour, baking powder and salt; add half to the butter mixture and beat on low speed just until combined. Add the milk and beat just until combined, then add the remaining flour mixture and beat just until combined.
4Divide the batter between 12 paper-lined muffin cups. Spoon the Nutella into a small microwave-safe dish and microwave it for20 seconds, until softened but not liquefied. 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.