Preheat the oven to 325°F and toast the hazelnuts for 11-15 minutes until fragrant.

If using unblanched hazelnuts, rub them between a kitchen towel to remove the skin.

Chop 75g of the toasted hazelnuts if adding to the dough.

Melt the chocolate if desired, then pour and spread it into a thin sheet to break into shards.

Prepare the hazelnut chocolate spread by dolloping portions onto a lined baking sheet.

Blend the cornstarch and 175g of toasted hazelnuts to make hazelnut flour.

In a bowl, combine the hazelnut flour, cornstarch, all-purpose flour, cake flour, baking powder, and baking soda.

In a separate bowl, beat the butter, brown sugar, white sugar, corn syrup, and salt until light and fluffy.

Add the egg, egg yolk, and vanilla extract, mixing until combined.

Gradually add the flour mixture and mix until just combined.

Incorporate the chocolate shards and chopped hazelnuts if using.

Portion the dough into 72-75g balls and flatten each ball to add hazelnut chocolate spread if desired.

Wrap the baking sheet in cling film and refrigerate for 12-24 hours.

Preheat the oven to 350°F and bake the cookies for 8 minutes, then rotate and bake for another 6-7 minutes.

Let the cookies cool for at least 5 minutes before enjoying.