Preheat oven to 350F. Line muffin pan with paper liners or place standing baking cups on a baking sheet. Set aside. 

In a large mixing bowl, whisk together flour, baking soda, baking powder, cinnamon, nutmeg and salt. Set aside. 

In the bowl of a standing mixer fitted with the paddle attachment, beat the butter, sugar and brown sugar on medium speed until smooth and creamy. Add bananas and mix until combined. Add egg, egg yolk and vanilla extract then continue mixing until combined. Use a rubber spatula to scrape down the sides and bottom of the bowl as necessary. 

Add half of the dry ingredients, mixing until mostly incorporated. Add half of the buttermilk, mixing until mostly incorporated. Repeat with remaining dry ingredients and buttermilk and mix just until smooth. 

Evenly distribute between cupcake liners, filling about 3/4 of the way full. Bake for 17 – 22 minutes, until a toothpick inserted in the middle comes out clean. Let cool in muffin pan for a few minutes, then transfer to wire rack to cool completely before frosting. 

In the bowl of a standing mixer fitted with the paddle attachment, beat butter until smooth and creamy. Add powdered sugar, cocoa powder, heavy cream, vanilla and salt. Beat until combined. Use a rubber spatula to scrape down the edges and bottom of the bowl as needed. Beat on medium-high for one more minute, then transfer to a piping bag to decorate.