In a large bowl or in the bowl of your stand mixer, add the butter, sugar, sea salt and vanilla. Beat well until smooth, about 3 minutes. Add the flour and hazelnut flour and beat on medium to high speed until the dough comes together.
Turn the dough out onto a well floured surface and form into a disk about 3/4 an inch thick. Wrap the dough in plastic wrap and place in the refrigerator for at least two hours.
Once chilled, line a large baking sheet with parchment paper. Remove the dough from the refrigerator and let the dough sit to warm slightly so it's easier to work with. Add more flour to your work surface and a rolling pin and roll out the dough to about 1/4 an inch thick, being careful to make sure all the cookies are the same width for even baking.
Cut the cookies our using a circular cookie cutter and transfer to the prepared baking sheet. Gather cookie dough scraps and re-roll and cut out more cookies until all the dough is gone.
Place the cookie dough back into the fridge for about 25 minutes. While chilling, preheat the oven to 325°F.
Bake the cookies from 20-25 minutes until they're golden brown around the edges. Remove from oven and allow to cool on pan for 2 minutes then transfer to a wire rack to cool completely.
While cookies are cooling, make the Nutella ganache. In a large bowl, add the semi-sweet chocolate and Nutella and set aside. In a medium saucepan, add the heavy cream and heat on high until it just begins to bubble around the edges. Remove and pour heavy cream over the chocolate and allow to sit without stirring for about 2 minutes. Stir with a rubber spatula until completely combined then cover with plastic wrap and sit for approximately one hour to thicken.
Once thickened, place the ganache in a plastic bag or piping bag and pipe chocolate on top of one half of the cookie. Place the other half of the cookie on top. Repeat until all cookies are made in to sandwiches. Store in an air tight container for up to 3 days or in the freezer for up to 3 months.