Review all recipe notes and instructions before beginning.
Line a half baking sheet or large plate with parchment paper. Dollop 2 teaspoons of nutella onto the parchment. Repeat 12 times. Place in the freezer while you make the cookie dough.
Cream butter and brown sugar for 2-3 minutes on medium high.
Add the egg, banana, and vanilla. Mix on medium and completely incorporated.
In this order, add the oatmeal, flour, salt, cinnamon, baking soda, and cornstarch. Mix on low or by hand with a spatula until just combined.
Fold in chopped chocolate. Leave some chocolate aside for topping if desired. Chill the dough in the freezer for 30 minutes or in the fridge for 2 hour or overnight. Preheat the oven to 350 degrees when the dough is almost done chilling.
Using a quarter cup or 4 tablespoon (#40) cookie scoop. Fill the scoop about ½ of the way up with cookie dough.
Press the nutella into the dough. Fill the remainder of the scoop with dough.
Place each ball on a baking sheet lined with parchment paper or a silicone mat. Repeat to fill the sheet with 6 cookies. Top with remaining chocolate. Place the remaining dough and nutella back into the freezer.
Bake for 12-14 minutes. The edges will brown slightly and the middle will look slightly underbaked. Let cool for 5-10 minutes or until set on the baking sheet.
Place the set cookies on a cooling rack. Let the baking sheet cool on the counter or use a cold one to make the remaining 6 cookies.