Preheat oven to 350 degrees, line 2 baking sheets with parchment paper or baking mats.
In a large mixing bowl or the bowl of a stand mixer, combine the granulated sugar and butter, cream until light and fluffy (about 2 minutes). Add the mashed banana, egg and vanilla extract, mix just until combined.
In a separate mixing bowl, whisk together the flour, baking soda and salt. Alternate adding the dry ingredients and the buttermilk until a very soft dough forms. Using a standard size cookie scoop, drop the dough onto the prepared baking sheets. Place in the oven (one at a time) and bake for 10-12 minutes. The cookies should still be soft but the centers should spring back when touched. Remove from the oven and allow to cool for 30 minutes before frosting.
While the cookies are cooling, prepare the frosting by creaming the chocolate hazelnut spread and the cream cheese together, chill until needed. Spread over the cookies.
Toast the hazelnuts in a clean pan over medium-low heat until fragrant. Chop and sprinkle over the frosting.
Notes
-You can add chopped hazelnuts to the cookie dough as well, they do not need to be toasted.