Line a cookie tray with parchment paper or wax paper.
Drop spoonfuls of Nutella onto the lined cookie sheet. Each spoonful should be about 1.5 teaspoons of Nutella. I used about 1/3 to 1/2 of my medium cookie scoop. You'll need about 14-16 dollops in total.
Place the cookie sheet in the freezer. Keep in the freezer until you're ready to assemble the cookies.
Whisk together the flour, cornstarch, baking powder, baking soda and salt in a medium bowl. Set aside.
In a large bowl beat together the butter, brown sugar and granulated sugar until there are no longer lumps of butter.
Beat the egg and vanilla extract into the butter mixture until combined.
Turn off the mixer and add the flour mixture into the butter mixture. Do a few stirs by hand first, then mix together with the electric mixer starting on a low speed until combined. The dough should not feel sticky to the touch/stick to your fingers. If the dough sticks to your fingers, mix in 1 more tablespoon of flour.
Stir in 1/2 cup chocolate chips.
Cover the bowl with plastic and place in the fridge for about 30 minutes or up to 24 hours.
To assemble the cookie dough balls: Take the dough out of the fridge. If it's been in the fridge overnight, then let it sit on the counter for about 10 minutes first. Scoop about 1 to 1.5 tablespoons of dough and flatten slightly. Place 1 frozen Nutella spoonful on top of the cookie dough. Scoop out another 1 to 1.5 tablespoons of dough and place on top of the Nutella. Pinch the cookie dough around the sides of the Nutella. Then roll the dough into a ball. Repeat the process with the rest of the cookie dough. Keep the Nutella in the freezer as you go so that the spoonfuls don't start to soften or melt.
Place cookie dough balls on a plate, cover with plastic and pop in the freezer for another 30 minutes.
When ready to bake, preheat the oven to 375F (190C). Line cookie sheets with baking paper (not wax paper) or silicone baking mats.
Place cookie dough balls 2-3 inches (about 5-7 cm) apart on the lined cookie sheets.
Bake 1 sheet at a time in the preheated oven for about 11-13 minutes, or until the tops look just set. Remove from the oven.
Optionally, dot the top of each cookie with a few extra chocolate chips (from the reserved 1/4 cup). Cool cookies on the cookie sheet for at least 10 minutes before transferring to a wire rack to continue cooling.