Preheat the oven to 180C/350F/Gas 4, line an 8x8-inch pan with foil and spray with baking spray. Set aside.
Whisk together the flour, cornflour, baking soda, salt, and spices. Set aside.
Whisk together the butter and sugar until combined. Add the egg, vanilla, and pumpkin, and mix until combined. Fold in the dry ingredients.
Press half of the dough into the prepared pan, and bake for 8 minutes. Microwave the Nutella until softened, then pour onto the cookie layer and spread out evenly. Place the remaining cookie dough on top.
Bake for a further 15 - 20 minutes until golden brown on top. Allow to cool completely in the pan on a wire rack before drizzling with more Nutella and cutting into bars.
Notes
These bars can be stored in an airtight container at room temperature or in the fridge for up to 3 days. They also freeze well for up to 3 months. Thaw overnight in the fridge before serving.