Add the toasted hazelnuts to a food processor and pulse or blend until finely ground. Transfer them to a bowl with the flour and salt and whisk to combine.
In a separate large bowl, use a spatula to combine the soft butter and brown sugar until no butter clumps remain and the mixture is smooth. Alternately, use an electric mixer or stand mixer and mix on low.
Add the dry ingredients to the butter mixture and stir until the dough starts to come together and pulls away from the sides of the bowl. You may need to mix with your hands to combine. The dough should hold together in a ball, but be slightly crumbly.
Turn the dough out onto a piece of plastic wrap and press it gently into a disc. Wrap well and chill for 2 hours or overnight (Note 2).
Line two large baking sheets with parchment paper. Preheat the oven to 350°F.
Gently flour a clean work surface or turn the dough out onto a piece of parchment paper. Gently flour the surface of the dough and roll it out to 1/4-inch thick. Cut the dough out using a round cookie cutter, I used a fluted one. Re-roll the scraps as needed. Cut a smaller shape (i used a snowflake) out of the center of half of the cookies.
Carefully transfer the cookies to the prepared baking sheets. Bake one at a time in the preheated oven for 9-11 minutes, until the edges are slightly golden. Cool on the cookie sheets for a few minutes then transfer to a cooling rack.
Place the chocolate and Nutella in a large mixing bowl. Heat the heavy cream in the microwave until scolding, but not boiling, about 1 1/2 minutes. Pour the heavy cream over the chocolate and let stand for 2 minutes. Mix until thickened and the chocolate is melted. Chill in the fridge for 20 minutes (or in the freezer for 10). It should be thick and barely fall back into the bowl when you lift it out with a spoon (Note 3).
Transfer the ganache to a piping bag and cut off the tip. Pipe the ganache onto the flat side of the whole cookies (without the centers cut out). Sandwich the cookies with the cutout on top, pressing gently.
Store in an airtight container for up to 3 days. Baked cookies (with or without ganache) freeze well for up to 3 months. Thaw in the fridge and enjoy.