Instructions

• Preheat the oven to 350°F. Line two baking sheets with parchment paper and set aside.
• In a large mixing bowl, add 1 cup salted butter (softened) and 2/3 cup granulated sugar. Use a hand mixer or stand mixer to cream them together until light and fluffy, about 2 to 3 minutes. Scrape down the sides of the bowl.
• Add 1 tablespoon ground flax seed and 1 teaspoon vanilla to the creamed butter and sugar. Mix until incorporated and scrape down the bowl once more.
• Place a fine sieve over the bowl and sift in 2 cups all-purpose flour (spooned and leveled), 1/4 cup cocoa powder, and 1 teaspoon cornstarch. Use a spatula to fold the dry ingredients into the butter mixture until a thick dough forms. The dough may look shaggy at first but should come together into a non-sticky, firm dough.
• Fold 1/2 cup dry toasted and unsalted hazelnuts (roughly chopped) into the dough until evenly distributed.
• Using a 1 1/2-tablespoon scoop (or measuring spoon), portion the dough into 24 even balls. Roll each portion between your palms into a smooth ball and place 12 dough balls on each prepared baking sheet, spacing them evenly.
• Use a rounded 1-teaspoon measuring spoon to press an indentation into the center of each dough ball to form the “thumbprint.” Press firmly but not all the way through so each cookie has a well for the filling. If the spoon sticks, wipe it with a clean dry towel between presses.
• (Optional) Spread the remaining 1/4 cup additional dry toasted and unsalted hazelnuts (roughly chopped) across a large plate. Gently press the outer edge of each cookie into the chopped hazelnuts so pieces adhere around the rim, then return cookies to the baking sheet.
• Bake one sheet at a time (or both if they fit and bake evenly) for 10 to 12 minutes, until the bottom edges have set but are not darkened.
• Remove the baking sheet from the oven. The cookie centers may have puffed; use the same rounded 1-teaspoon measuring spoon to press each center again to restore the indentations.
• Let the cookies cool on the baking sheet for 10 minutes to firm up.
• Divide the 1/3 cup chocolate hazelnut butter among the 24 cookies (about 2/3 teaspoon per cookie) and spoon it into each cookie’s center. If using, place one halved hazelnut (from the 12–15 hazelnuts cut in half) into the center of each filled cookie.
• Let cookies cool completely before transferring to an airtight container. Store at room temperature for up to 3 days. Note: fill only the cookies you plan to eat soon, as filled cookies can stick together if stacked.