Use a ½ teaspoon to scoop a dollop of Nutella on top of each muffin and use a toothpick to swirl it in, don't over swirl, you want to have thick clumps of Nutella!
Bake for the first 5 minutes at the higher temperature then reduce the oven temperature to 350F/180C and bake for an additional 8-11 minutes or until a toothpick inserted in comes out clean.