Pre-heat oven to 400 degrees. Line cookie sheets with parchment paper. (Or if using foil, be sure to grease.)
Sift flour, sugar, baking soda and salt together in a large bowl.
In an electric mixer, cream butter and slowly add flour mixture. Beat in mashed banana and egg. Mix in oats. Take a knife and slice/swirl nutella into the batter. Do not fully mix to keep the swirls within the batter.
Using a cookie scoop, drop tablespoonfuls onto prepared baking sheet. Bake for 8-10 minutes, until lightly golden.
Note: Dough will not be your typical stiff cookie batter and be on the softer side of things.
Cool on baking sheet for 5 minutes before removing and cooling completely on a wire cooling rack.
Recipe by Sweet Dash of Sass at http://sweetdashofsass.com/2014/12/banana-nutella-cookies/