Mix the dry ingredients

• Whisk together the flour, baking soda, baking powder and cornstarch in a bowl.
• Set aside.

Mix the wet ingredients

• Whisk the melted and cooled butter with the dark brown sugar and granulated sugar until combined.
• Add the cold egg, egg yolk and vanilla bean paste.
• Whisk until smooth and glossy.

Make the cookie dough

• Gradually fold the dry ingredients into the wet mixture until just combined, without overmixing.
• Fold in the semisweet chocolate chips, reserving some for the top.

Assemble the pie

• Grease and line an 8-inch round baking pan.
• Press about half the cookie dough into the bottom, bringing it slightly up the sides.
• Spread the Nutella in the center, leaving a border around the edges.
• Cover with the remaining cookie dough and seal the edges completely so the Nutella doesn't leak out, then press the remaining chocolate chips on top.

Bake

• Bake at 175 °C until the edges are golden brown and the center looks slightly soft.

Cool before slicing

• Let the cookie pie cool in the pan so the center sets while the cookie stays soft and chewy.
• Slice and serve.