Chocolate Truffle Thumbprint Cookies

Center a rack in the oven and preheat to 350 (F). Line two sheet pans with parchment paper and set aside.
In a medium bowl, whisk 2 1/4 cups all-purpose flour, 1/3 cup natural unsweetened cocoa powder, and 3/4 teaspoon kosher salt until combined.
In the bowl of a stand mixer fitted with a paddle attachment, combine 1 cup unsalted butter and 2/3 cup granulated sugar. Beat on medium speed until light, fluffy, and doubled in volume, about 3 to 5 minutes, using a rubber spatula to scrape down the bottom and sides of the bowl occasionally. Reduce the mixer speed to low and add 1 large egg white and 1 teaspoon pure vanilla extract, mixing until just combined. Scrape down the bottom and sides of the bowl with a rubber spatula.
With the mixer on low, gradually add the dry ingredients (from the 2nd step). Increase the mixer to medium-low and beat until the dough clumps around the paddle and/or sides of the bowl, about 2 to 3 minutes. Scrape down the sides of the bowl with a rubber spatula.
Use a 1-tablespoon cookie dough scoop to portion out 28 cookies. Use your hands to roll each cookie into a ball and press your thumb into the center of each cookie. Place the cookies on the prepared sheet pans, leaving at least 1 1/2-inches between each cookie. Bake each sheet pan for 16 to 18 minutes, or until they feel firm to the touch. Cool the cookies on their sheet pans on a wire rack to room temperature completely before assembling.

Use a fine-mesh sieve to sprinkle confectioners' sugar over the cookies.
Place 2 ounces roughly chopped dark chocolate in a medium, heatproof bowl. Microwave on medium for 15 second intervals, using a heatproof rubber spatula to stir occasionally, until chocolate is completely melted. Once melted, cool chocolate in its bowl on a wire rack for 5 minutes, stirring occasionally to release heat.
After 5 minutes, the chocolate should be warm and liquid, but not hot. Working quickly spoon a small dollop of chocolate into the indentation of each cookie. Press a truffle into the chocolate. Refrigerate for 5 minutes to allow the chocolate to set, and serve immediately. The cookies can be kept at room temperature, in an airtight container or Ziploc bag, for up to 3 days.

Get the Recipe: Chocolate Truffle Thumbprint Cookies