- 3/4 cup all-purpose flour
- 1/4 teaspoon baking soda
- 1/4 teaspoon cream of tartar
- 1/4 teaspoon salt
- 1 stick unsalted butter, softened
- 3/4 cup sugar
- 1/2 teaspoon finely grated orange zest
- 1/2 teaspoon pure vanilla extract
- 2 large egg whites, room temperature, lightly beaten
- 1/2 cup finely chopped toasted hazelnuts
- 1/4 cup quick-cooking oats
- 8 ounces bittersweet chocolate, finely chopped
- Preheat oven to 350 degrees. Whisk together flour, baking soda, cream of tartar, and salt. Beat together butter, sugar, and orange zest with a mixer on medium-high speed until light and fluffy, about 3 minutes. Add vanilla, then egg whites, and beat on medium-high speed until well combined, about 3 minutes. Stir in flour mixture, then nuts and oats.
- Transfer mixture to a pastry bag fitted with a 1/2-inch plain round tip. Pipe into seventy two 2-inch-long fingers, spacing them 3 inches apart, on baking sheets lined with nonstick baking mats or parchment. Bake until lightly golden with darker edges, 12 to 15 minutes. Transfer cookies to wire racks with a metal spatula, and let cool completely.
- Melt chocolate, stirring, in a bowl set over a pan of simmering water. Spread 1 teaspoon chocolate each onto flat side of half the cookies. Sandwich with remaining cookies, pressing gently.
Cookies can be stored at room temperature in an airtight container up to 2 days.