For the cupcakes

1. Preheat the oven to 350°F. Line a 12-cup cupcake tin with paper liners.
2. In a medium bowl, sift together the flour, baking powder, baking soda, and salt. Set aside.
3. In a large bowl, beat the butter and granulated sugar with a handheld mixer or stand mixer until light and fluffy, about 2 minutes. Add the sour cream and vanilla. Slowly beat in the eggs one at a time.
4. Fold in the dry ingredients until thoroughly combined. Add the mashed bananas and mix until just combined — do not overmix.
5. Using an ice cream scoop, fill each cupcake liner three-quarters full. Bake until the tops are golden and a toothpick inserted in the middle comes out clean, 18–20 minutes. Cool on a wire rack completely before frosting.

For the frosting

1. Beat the butter on medium speed until light and creamy, about 2 minutes. Add the chocolate-hazelnut spread and beat until smooth.
2. Add the powdered sugar in two additions, beating well after each. Add the heavy cream or milk one teaspoon at a time until the frosting is light and pipeable. Add a pinch of salt and beat briefly to combine.
3. Pipe or spread the frosting onto the completely cooled cupcakes. A large French star tip gives a beautiful swirl effect.