Into a big mixing bowl, add the room temperature butter, and cream it with an electric hand mixer for 2-3 minutes or until it's creamy and light in color.
Add the brown sugar, and granulated sugar. Cream the butter and sugar with an electric hand mixer until the mixture is creamy and light. Add the eggs, one at a time, and vanilla extract, and mix until combined and creamy.
Then, add the flour, cake flour, cornstarch, baking soda, and kosher salt, and mix with the hand mixer until almost combined.
Add the chocolate chips and fold with a spatula until the mixture is just combined.
Cover the bowl with plastic wrap and let the dough rest in the fridge for at least an hour. We let the dough rest in the fridge overnight.
Freeze the Nutella by scooping about 2 teaspoon of Nutella on a parchment paper-lined baking sheet and place in the freezer for at least an hour.
Tip: Nutella melts very fast, so when you are making the stuffed cookie dough balls, keep the Nutella in the freezer.
Use a cookies scooper to scoop out the cookies or scoop the cookies with a regular tablespoon, 2 tablespoons at a time.
Place the cookie dough balls onto a parchment paper-lined baking sheet and flatten the balls into disks using the palm of your hand. Place the frozen Nutella on the dough disk and roll it into a ball.
Tip: to prevent these cookies from spreading too much, place the cookie dough balls in the freezer for 30 minutes before baking.
Bake in a 180c (350f) preheated oven for 11-12 minutes. Let the cookies cool for 5 minutes, and then transfer them to a wire rack to cool completely.