Freeze the Nutella: Line a plate with parchment. Scoop Nutella into 12 small mounds (about 2 teaspoons each).Freeze until firm, at least 30–45 minutes. This makes stuffing easier and prevents leaks. Preheat and prep: Heat the oven to 350°F (175°C). Line two baking sheets with parchment paper or silicone mats.
Mix dry ingredients: In a medium bowl, whisk flour, baking soda, baking powder, salt, and cinnamon.Set aside. Cream butter and sugars: In a large bowl, beat the butter, granulated sugar, and brown sugar until light and fluffy, about 2–3 minutes. Scrape down the bowl as needed.
Add egg and vanilla: Beat in the egg and vanilla until smooth and glossy.
Fold in the banana: Mash the banana until nearly smooth with a few small bits. Stir it into the butter mixture just until combined.The batter will look slightly curdled—that’s okay. Add dry ingredients: Add the dry mixture to the wet and mix on low until just combined. Do not overmix. If using chocolate chips, fold them in now.
Chill briefly: Cover and chill the dough for 20–30 minutes. This helps prevent spreading and makes shaping easier since banana adds moisture.
Shape the cookies: Scoop about 2 tablespoons of dough and flatten it into a disk in your palm.Place a frozen Nutella mound in the center. Top with another 1 tablespoon of dough and seal the edges well, rolling gently to form a ball. Repeat with remaining dough and Nutella. Bake: Arrange cookies on prepared sheets, spacing them a few inches apart.Bake 10–12 minutes, until the edges are set and lightly golden but the centers still look soft. Do not overbake or you’ll lose the gooey center. Cool: Let cookies rest on the baking sheet for 5 minutes, then transfer to a wire rack to cool. They’ll continue to set as they cool.
Serve warm: These are best slightly warm when the centers are melty. If cooled, rewarm in the microwave for 10–12 seconds.