Start by placing 10 x 20g dollops (heaped tablespoon) of Nutella on a small baking tray that's lined with baking paper. Place in the freezer for 1 hour or until frozen.
In a medium bowl, lightly soften the cubed butter. Note: you do not want the butter to be creamed or reach room temperature, so mix the butter using a wooden spoon, electric whisk or stand mixer just until the cubes have been broken up.
Add white and brown sugar, and mix until just combined.
Add the mashed banana and vanilla essence and mix until mostly combined. Note: the mixture will slightly separate so don't worry.
Add the flour, baking powder and salt. Mix until there are no more flour pockets remaining.
Add the chocolate chips and mix until just combined. Place in the cookie dough in the fridge for 30 minutes.
Remove the cookie dough from the fridge and evenly separate the cookie dough in 10 balls (weighing around 110g each). Lightly flatten the dough and place a frozen dollop of Nutella in the center and seal, ensuring the cooking dough has jagged edges. Note: you don't want to the cookie drought o have smooth edges.
Once all the cookie balls have been stuffed with Nutella, cover the cookie tray with gladwrap (without flattening the dough) and freeze for a minimum of 90 minutes. Note: do hot overcrowd the cookie tray when freezing. Also, the cookie dough can be frozen for up to 2 months.
Preheat the oven to 205℃/400℉ (no fan) 30 minutes prior to baking the cookies. Line a medium baking tray with baking paper.
Place 3-4 cookies on the prepared baking tray. The cookie with spread so allow a good amount of space in between. Bake for 18 minutes. Allow to slightly cool before digging in. Note: best enjoyed when freshly baked but I'm not that picky - it's always a the right time for a chunky choc banana cookie.