Whisk together 240 grams of hazelnut or almond flour, 1 cup of all purpose flour, 2/3 cup of sugar, 1/4 teaspoon of salt and 4 tablespoons of cornstarch in the bowl of a stand mixer
Using the paddle attachment, add 1 tablespoon of vanilla extract and incorporate 10 tablespoons of cold, cubed butter, until a wet sandy textured dough forms and you no longer see chunks of butter
Form 1 tablespoon balls of dough and place them onto parchment lined baking sheets
Refrigerate the shaped dough for 2 hours
Preheat the oven to 325 degrees
Bake chilled dough for about 18 minutes
Transfer cookies to a cooling rack until fully cooled
Melt 1/2 cup of dark chocolate morsels and stir in 1/2 teaspoon of espresso powder until smooth
Dollop 1/2 teaspoon of meted chocolate in between two matched fully cooled cookies
Chill the sandwiches until the chocolate is set