Preheat your oven to 350°F (175°C).

In a large mixing bowl, whisk together the flour, cocoa powder, powdered sugar, and salt.

Add cold butter cubes to the dry mixture and mix until it resembles coarse crumbs.

Stir in the egg yolk and gradually add cold water until the dough comes together.

Wrap the dough in plastic wrap and refrigerate for about 30 minutes.

Roll out the dough to about 1/4 inch thick, cut circles, and press into tartlet pans.

Prick the bottoms with a fork and bake for 12–15 minutes. Let them cool completely.

Heat the heavy cream in a small saucepan until just simmering.

In a separate bowl, combine hazelnut spread, cocoa powder, and vanilla extract. Pour the hot cream over and stir until smooth.

Fill the cooled tartlet shells with the chocolate hazelnut filling.

Refrigerate for at least 1 hour to allow them to set.

Before serving, sprinkle with chopped hazelnuts.

Serve chilled or at room temperature.