Preheat oven to 325 degrees F. Line 12 muffin tins with paper liners.
Cream together butter and sugar until light, 2 minutes. Add in eggs one at a time, until fully incorporated. The batter may not appear smooth. Add vanilla. Stir in flour, salt and baking powder until batter is uniform and no flour remains.
Using an ice cream scoop for uniformity, fill each muffin liner ¾ full with the batter. Top each cake with 1 ½ teaspoons of Nutella. Swirl Nutella with a toothpick or sharp knife, making sure to fold a bit of the batter up over the Nutella.
Bake in preheated oven for 20 minutes. Allow to cool completely.