Place the hazelnuts with 2 tablespoons of the granulated sugar in a food processor. Process until the hazelnuts are finely ground. (leave in food processor).
Combine the remaining sugar, flour, a pinch of salt and the softened butter in a medium bowl.
Use your fingers or a pastry cutter to work this mixture just until the butter is evenly distributed and resembles clusters of streusel topping.
Add the flour and butter mixture to the hazelnut mixture in the food processor. Pulse till the dough comes together in a dense mass.
Divide the dough into 4 equal portions.
Wrap the dough in saran wrap and refrigerate for a minimum of 2 hours. (I chilled my dough for 4 hours). The dough should be very cold and firm.
Line 2 cookie sheets with parchment paper.
Unwrap the disk of dough and cut 1/2 inch strips.
Working with one strip at a time, cut the strip into 1/2 inch pieces of dough and roll each piece into smalls balls about the size of a hazelnut in its shell.
Place the balls on the parchment lined cookie sheet while spacing them about 2 inches apart.
Once you have filled one cookie sheet, place it in the fridge and repeat with the remaining cookie balls.
Chill the rolled cookies for about 1 hour or if pressed for time freeze for 1/2 hour.
When ready to bake, preheat the oven to 300 degrees F.
Bake one sheet at a time for about 13 minutes. They should look light golden brown in colour.
Place the cookie sheet onto a cooling rack and let cool completely. Repeat with the second cookie sheet.
Let the baked cookies cool completely before filling with the melted chocolate.
When ready to assemble the cookies place the dark chocolate chips in a microwave safe bowl and melt it on high for about one minute stopping to stir at 30 second intervals.
Use a small spoon or fill a piping container with the melted chocolate and put a dollop of chocolate on the flat side of a cookie.
Top (flat side down) with a second cookie while pressing down slightly to form a sandwich.
Repeat with the remaining cookies.
Store in a cool dry place.