Preheat the oven to 350°F (175°C) and line a baking sheet with parchment paper.

In a large mixing bowl, combine 1 cup of Nutella, the all-purpose flour, and the egg.

Mix the ingredients together using a wooden spoon or electric mixer until a stiff, uniform dough forms.

Roll the dough into 24 equal-sized balls, approximately 1 inch in diameter.

Place the balls on the prepared baking sheet and flatten slightly with the palm of your hand or the bottom of a glass.

Bake for 8 to 10 minutes, or until the edges are set but the centers remain slightly soft.

Remove from the oven and allow the cookies to cool completely on a wire rack.

Once cooled, spread approximately 2 teaspoons of the remaining Nutella onto the flat side of 12 cookies.

Top with the remaining 12 cookies to create sandwiches.