Nutella Sandwich Cookies

1. Preheat the oven to 275°F. Line two baking sheets with parchment paper.

2. Make the Meringue: In the bowl of an electric mixer fitted with the whisk attachment, whisk the egg whites and sugar to combine. Remove the bowl and place it over a medium pot of simmering water. Warm the mixture, whisking constantly by hand, until the sugar has dissolved, 1 to 2 minutes.

3. Transfer the bowl back to the electric mixer. Add the cream of tartar and the salt, and whip on medium-high speed until the egg whites are thick and can hold a peak. Add the vanilla extract and beat just until combined.

4. Bake the Meringue: Using two spoons, drop 2-tablespoon dollops of meringue onto the prepared baking sheets. They don’t need to be perfect, but should be around the same size. Bake until the meringues are crisp but not golden, about 1½ hours. Cool completely. (The meringues can be made a day or two ahead and stored in an airtight container.)

5. Assemble the Sandwich Cookies: Spoon 1 to 2 teaspoons chocolate-hazelnut spread onto the flat side of a meringue cookie. Top with another meringue cookie and gently press to seal (not too hard, as the meringues are a bit fragile). Repeat with the remaining cookies and chocolate-hazelnut spread. Serve immediately.

Note: The meringues keep better un-sandwiched, so it’s best to assemble the cookies the day you're serving them.