Instructions

1. Preheat oven to 375°F.
2. Mix 1/2 cup Nutella and the egg with an electric mixer until well combined. Slowly add 1 cup of flour until a wet dough is formed. Transfer the dough to a flour-dusted board.
3. Knead gently, adding a bit more flour if necessary; dough will be sticky. Roll dough into 18 balls, flouring your hands as needed to make rolling easier. Place on parchment-lined baking sheet, several inches apart.
4. Bake 12 minutes. Let cool 10 minutes. Split cookies in half horizontally using a sharp, serrated knife (see note below). Spread bottom with 1 teaspoon Nutella, replace top and press firmly. Let cool completely. Store in a tightly covered tin.
5. Note on flour: I didn't have self-rising cake flour, so I mixed 1 1/2 teaspoons baking powder and 1/2 teaspoon salt with regular cake flour before adding it.
6. Note on splitting the cookies: I had a problem with a couple of the cookie tops breaking a bit when I first started cutting them. I then tried it this way and had no more problems: turn the cookie on its side and carefully score the entire outside edge while turning it like a wheel, then gently cut the rest of the way through the middle.