In a medium bowl combine all-purpose flour, almond flour, cornstarch, and salt. Whisk together, and set aside.

In a large bowl, combine butter and sugar. Using an electric mixer fitted with the beater attachment, or a wooden spoon by hand, cream together until well combined and slightly lightened in color (about 2 minutes using an electric mixer set to medium speed or 3 minutes by hand).

Whisk the egg and vanilla into your butter / sugar mixture until combined. Add the dry ingredients, and mix together until you have a thick, sticky dough.

Divide the dough in half and wrap each half with plastic wrap. Flatten into a rough disk shape, and chill in the fridge for 1 to 2 hours, or until firm.

Take one of the disks of dough out of the fridge, unwrap and place on a lightly floured work surface. Roll the dough out until it's between ¼ inch and ⅛ inch thick. Use a 2 & ½ inch round cookie cutter to cut, and transfer cut cookies onto your parchment lined baking sheet. You don't need a whole lot of space between the cookies - they don't spread much - but leave a little bit of room for air to circulate (at least ½ an inch between each cookie).

Gather up the dough scraps and continue rolling and cutting until the first half of dough is used up. If the dough becomes too soft to work with at any point, wrap it in plastic wrap, and place in the fridge for 15 to 20 minutes, or until firmed up again before continuing to roll.

Repeat the rolling and cutting process with the second half of dough, but this time, cut out a center hole using a smaller cookie cutter to create a little window in the cookies.

Pop the cookies into the freezer for 10 minutes to firm up before baking. Bake the cookies in the center of your preheated 350°F oven for 12 to 15 minutes, or until golden brown along the edges. The cookies with a center cutout will take less time to bake. Start checking on the cookies around the 10 minute mark.

Cool the cookies on the baking sheets for 5 minutes, then transfer onto a wire rack, and set aside to cool completely to room temperature.

Spread a small amount of Nutella on the bottom of a cookie (without a center cutout) and sandwich with a cookie with a center cutout. Repeat until all of the cookies are sandwiched together.