Freeze the Nutella portions

• Scoop small portions of Nutella onto a tray lined with parchment.
• Freeze until firm so they are easy to wrap in the dough.

Cream butter and sugars

• Beat the softened butter with the granulated sugar and brown sugar until light and fluffy.
• Add the egg, egg yolk and vanilla and beat until smooth.

Mix the dry ingredients

• Whisk the flour, cornstarch, baking soda and salt together in a bowl.

Make the cookie dough

• Add the dry ingredients to the butter mixture and mix just until combined.
• Fold in the chopped milk chocolate, chocolate chips and toasted walnuts.

Stuff and shape the dough balls

• Flatten a portion of dough in your hand, place a frozen Nutella portion in the center, then seal and roll into a ball.
• Repeat with the remaining dough and Nutella.

Freeze the dough balls

• Place the stuffed dough balls on a tray and freeze until firm.

Bake

• Preheat the oven to 175 °C (350 °F).
• Arrange the dough balls on a baking tray, spaced apart, and bake until the edges are set and the centers still soft.

Cool

• Let the cookies rest on the tray for a few minutes, then transfer to a rack to cool.