"Try these traditional Italian cookies from Siena for a deliciously different Christmas treat. They're often given as gifts, but you'll definitely want to keep some of these almond treats for yourself before you give the whole lot away!"
Combine white sugar and almonds in the bowl of a food processor; pulse into a fine powder. Pour into a bowl.
Combine 1/4 cup egg whites and vanilla extract in a bowl; add to almond-sugar mixture, mixing as little as possible. Wrap dough in plastic wrap and refrigerate for 12 hours.
Line a baking sheet with parchment paper.
Remove dough from fridge. Sift 2 tablespoons confectioners' sugar and flour onto a clean work surface. Place dough in the center and add remaining 1/4 cup egg whites. Knead egg whites into dough until soft and smooth, adding additional confectioners' sugar and flour as needed.
Roll portions of dough into 1-inch-thick logs. Take walnut-sized dough pieces from each log and shape into diamonds with slightly wet hands. Place cookies on the prepared baking sheet and generously dust with confectioner's sugar. Let rest at room temperature for 1 to 2 hours.
Preheat oven to 350 degrees F (175 degrees C).
Bake cookies in the preheated oven until lightly golden, 7 to 8 minutes. Cool on the baking sheet before serving.