For the Cookies

1. Cream butter, brown sugar, and granulated sugar on medium low speed in a stand mixer fitted with a paddle attachment.
2. Add egg, yolk, melted chocolate, and vanilla. Mix until incorporated.
3. Sift dry ingredients and add to dough. Stop mixing as soon as the dough comes together!
4. Roll out dough on a piece of wax paper and put it on a half sheet pan. The dough should be between 1/8 and 1/4 inch thick. Cover and then refrigerate for 1 hour.
5. Next, use a circle cutter around 2 inches to cut out cookies. Transfer the cut out cookies to parchment lined baking trays, and refrigerate for 10 more minutes.
6. Preheat oven to 350 degrees F, and bake cookies for 7-9 minutes. Be sure not to over bake the cookies! Even one minute can mean the difference between super soft or hard edges. I bake mine right at 8 minutes, but even 7 1/2 would be great.
7. Transfer baked cookies to a cooling rack as soon as they come out and let cool completely before making cookie sandwiches.

For the Chocolate Hazelnut Filling

1. Cream softened butter and hazelnut spread together. Add powdered sugar and mix until combined. Thin with milk as is needed. Start with just 1 tablespoon, and then add more if the frosting is too thick. 
2. Prepare piping bag with desired piping tip and hazelnut filling. Be sure to rubber band or clamp the top of the bag so the filling doesn’t come out!
3. Pipe the filling on one cookie, then top with another and repeat until all cookies sandwiches are complete.