Preheat oven to 325°F Line 12 muffin tins with paper liners.
Cream together butter and sugar until light and fluffy,about 2 minutes. Add in eggs one at a time, until fully incorporated. Don’t worry if the batter doesn’t look smooth. Add vanilla. Stir in flour, salt and baking powder until batter is uniform and no flour remains.
Using an ice cream scoop or spoon , fill each muffin liner with batter. They should be 3/4 full, if you’re not using a scoop. Top each cake with 1 heaping teaspoon of Nutella. Swirl Nutella in with a toothpick, making sure to fold a bit of batter up over the nutella.
Bake for 20 minutes or until a cake tester comes out clean.