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. I got 13 cookies with this recipe. I recommend dropping a few extra spoonfuls of Nutella in case it yields a few more cookies for you.
Place the cookie sheet in the freezer for about 10-15 minutes. Then remove from the freezer and flatten the spoonfuls down slightly so that they are round circles. Place back in the freezer for 20-30 minutes. Keep in the freezer until you're ready to assemble the cookies.
Preheat the oven to 350F (180C or 170C on a fan forced oven).
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 in the egg and vanilla extract.
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.
Stir in the chocolate chips.
Line cookie sheets with baking paper (not wax paper) or silicone baking mats.
Scoop about 1 to 1.5 tablespoons of dough and flatten slightly.
Place 1 disc of frozen Nutella in the middle of the cookie dough.
Scoop another 1 - 1.5 tablespoons of cookie dough and place on top of the Nutella disc.
Pinch the dough around the edges of the Nutella and roll the cookie dough into a ball shape. Be careful to keep the Nutella horizontal.
Place cookie dough balls 2-3 inches (bout 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. Cool cookies on the cookie sheet for at least 10 minutes before transferring to a wire rack to continue cooling. Note: I recommend placing cookie dough balls in the fridge as the first tray is baking.