With a stand mixer, mix the hazelnut flour, the all-purpose flour, the sugar, and the softened butter but be careful not to melt it. Mix until the dough is smooth and holds together nicely.
Wrap it in plastic wrap and let it stand in the fridge for about one hour. When solid, it is easier to shape;
Divide the dough into walnut-sized balls and place them on a parchment-lined baking sheet.
Don’t make overly big cookies that won’t cook evenly in the oven. They will be raw and chewy inside but crispy outside. Good cookies are crunchy inside and outside.
Take a cutting board and press it down on the cookie balls in order to flatten them. If you think it’s faster, take a drinking glass and flatten each cookie individually.
Bake in a static oven at 350°F (180°C) for about 12-15 minutes. Take them out when they just start to turn golden. Let them cool while you prepare the chocolate filling;
Gently melt the chocolate in a bowl set over a pan of simmering hot water.
When the chocolate is melted, you are ready to assemble the cookies. Put a dollop of chocolate on the flat surface of a cookie—you may use a teaspoon or a pastry bag—then place another cookie on top. Put the cookies on a wire rack and let them cool down before serving.