Make the pastry base:
In a bowl, rub the cold cubed butter into the flour until it resembles breadcrumbs. Stir in the powdered sugar. Add the egg yolk, vanilla extract, and 1 teaspoon cold water. Mix until a dough forms. Add an extra teaspoon of water if needed. Shape into a disk, wrap in plastic, and chill for 30 minutes.

Prepare the cookie dough:
In a large bowl, whisk together melted butter, caster sugar, and light brown sugar until smooth. Beat in the egg and vanilla extract. In a separate bowl, sift together flour, baking powder, cornflour, baking soda, and salt. Gradually fold the dry ingredients into the wet mixture until combined. Stir in the chocolate chips.

Preheat the oven:
Set oven to 180°C (350°F) and line baking trays with parchment paper.

Assemble the cookies:
Roll out the pastry dough on a lightly floured surface to about ¼ inch thick. Cut into 12 circles (about 2.5-3 inches in diameter). Scoop 1 tablespoon of cookie dough, flatten slightly in your palm, place 1 teaspoon of Nutella in the center, then cover with a second flattened tablespoon of cookie dough. Seal edges gently.

Wrap with pastry:
Place a pastry circle on a flat surface, put the filled cookie dough ball in the center, and wrap the pastry around it, pinching the edges to seal completely. Place on the baking tray, seam side down. Repeat with all cookies.

Bake:
Bake for 12-15 minutes until golden brown. The pastry should be crisp and the cookie dough cooked through.

Cool and serve:
Allow the cookies to cool on the tray for 5 minutes before transferring to a wire rack. Serve warm to enjoy gooey Nutella centers.