Hazelnut Cookies 

1. In a large mixing bowl, whisk together the flour, hazelnut flour, and salt. With a stand or hand mixer, cream the butter and brown sugar together at medium-high speed for 2-3 minutes or until the mixture appears smooth and fluffy.
2. Beat the vanilla extract into the butter mixture followed by the egg at medium-high speed. Stop the mixer and use a silicone/rubber spatula to scrape the sides and base of the mixing bowl.
3. Mix the dry ingredients into the wet batter at low speed just until cookie dough comes together and starts to form into a ball. Take care to not overmix.
4. Transfer the cookie dough to a lightly floured work surface and divide the dough into equal portions. Shape each portion into a flat disk.
5. Wrap both disks of dough in plastic wrap and refrigerate for 2-3 hours or overnight. If disks of dough are chilled for more than 3 hours, allow the dough to rest at room temperature for 10-12 minutes to soften up a little before rolling and cutting into circles.
6. Line (2) large baking sheets with parchment paper or silicone baking mats. Working with one disk of dough at a time, lightly flour a work surface and roll the cookie dough into a large circle about 1/4 inch thick.
7. Using a 2 inch circular cookie/biscuit cutter lightly dipped in flour, cut out cookie dough pieces and transfer to the prepared baking sheet(s) placing each piece of dough 1 inch apart.
8. Re-roll the cookie dough scraps and cut as many circle shapes as you can. Repeat the process with the remaining disk of cookie dough. You should end up with 54 cookie dough pieces divided between the two baking sheets.
9. Preheat the oven to 350F(175C). Place cookie dough cutouts in the freezer for 15 minutes or in the fridge for 30 minutes before baking. This will allow the butter in the dough to firm back up and prevent the cookies from overspreading and losing their shape once baked.
10. Bake the cookie dough cutouts for 12-14 minutes or until the edges appear crisp and the top of the cookies no longer look wet (unbaked). If your oven runs hot, check on the cookies between the 10-12 minute mark.
11. Cool the cookies on the baking sheet(s) for 3-4 minutes before transferring to cooling racks to cool completely to room temperature.

Assembly

1. Add the chopped chocolate and oil in a small heat-proof bowl and place over a pot of simmering water (Make sure the bowl is not touching the water). Intermittently mix the chocolate and oil together until it melts into a smooth mixture.
2. Carefully remove the melted chocolate from the heat and set it aside to cool. Fill a large pastry bag with the chocolate hazelnut spread. Set aside.
3. Turn over half of the cooled cookies. Snip off about 1/8 to 1/4 inch from the tip of the pastry bag and pipe a generous amount of the spread into the center of the cookies. Refill the pastry bag with more spread as needed.
4. Top the cookies with the remaining cookies to form a sandwich. Drizzle the cooled chocolate over the top of the cookie sandwiches and sprinkle with the chopped hazelnuts (optional) before serving.