In a large mixing bowl, beat 1 cup of unsalted butter and 2/3 cup of granulated sugar together using an electric mixer on medium speed until the mixture is light and fluffy—about 3-4 minutes. Add in 1 egg yolk and 1 teaspoon of vanilla extract, blending well until fully incorporated.

In a separate bowl, whisk together 1 1/2 cups of all-purpose flour, 2/3 cup of almond flour, 1/3 cup of cocoa powder, 1 tablespoon of cornstarch, and 1/4 teaspoon of fine salt. Gradually mix this dry mixture into the butter mixture, stirring until just combined. Be careful not to overwork the dough.

Divide the dough into two equal pieces and flatten each into a rectangular shape. Wrap them tightly in plastic wrap and chill them in the fridge for at least 45 minutes.

While the dough is chilling, preheat your oven to 350°F (175°C) and prepare your baking sheets by lining them with parchment paper.

Once chilled, remove the dough from the fridge and roll it out on a lightly floured surface to about 1/4 inch thick. Use a 1 1/2-inch cookie cutter to cut out shapes, and remember to use a smaller cutter for half of the cookies to create appealing cutouts.

Place the cookies on your prepared baking sheets, ensuring they are spaced about 1 inch apart. Bake in the preheated oven for 9-10 minutes, or until the edges are set but the centers are still soft. Allow the cookies to cool on the baking sheet for at least 15 minutes before transferring them to a wire rack.

Once the cookies have cooled, dust the tops of the cut-out cookies with 1/4 cup of powdered sugar. Spread a generous layer of Nutella on the solid cookies and sandwich them with the dusted cookies.