Preheat oven to 350 degrees and line 12 muffin cups with cupcake liners.
Combine flour, baking powder, baking soda, cinnamon, nutmeg, and salt in a medium bowl.
In another bowl, mix together mashed banana, sour cream, and vanilla.
Using an electric mixer, beat butter and sugar until light and fluffy, about 3 minutes. Add egg and egg yolk and mix well.
Add flour mixture in 3 additions, alternating with banana mixture in 2 additions.
Divide batter between prepared muffin cups and bake for about 20 minutes, or until just firm on top.
Let cool a few minutes in pan and then remove from pan to cool completely.
For frosting, beat butter until smooth. Add confectioners' sugar and beat on low until smooth. Add nutella and beat on medium until fully mixed in. Beat in half-and-half and vanilla. Frost cupcakes using a pastry bag fitted with a large tip or with an offset spatula.