Preheat your oven to 180°C (355°F) or 160°C (320°F) for fan-assisted baking and prepare baking trays with parchment paper.

Whisk together the all-purpose flour, Dutch processed cocoa powder, and salt until well combined. Set aside.

Cream together unsalted butter, powdered sugar, egg yolk, and milk using an electric mixer until light and fluffy.

Gradually mix in the flour and cocoa mixture until just combined. Fold in the finely chopped hazelnuts.

Using an ice cream scoop, portion out about 1.5 tablespoons of dough, roll into balls, and coat with chopped hazelnuts.

Bake in the preheated oven for about eight minutes until they are soft but set.

Once cooled slightly, create an indentation in the center of each cookie using a tablespoon.

Fill the indents with chocolate hazelnut spread using a spoon or piping bag. Allow to set before serving.