1. Sift together the flour, sugar, and baking soda and set aside.
2. Put the almond flour, butter, and salt into the bowl of an electric mixer fitted with the paddle attachment and beat on medium speed until the mixture resembles cornmeal, about 3 minutes. With the machine running, add the egg and almond extract and mix until well incorporated. Turn the speed to low and add the flour mixture. Mix just until no traces of flour remain.
3. Transfer the dough to a large sheet of plastic wrap, flatten into a 1-inch-thick disk, and wrap tightly in the plastic. Refrigerate until hard, at least 30 minutes or up to 3 days.
4. When ready to bake, preheat the oven to 325°F. Line two baking sheets with parchment paper.
5. Form the dough into ½-inch balls and put 1 inch apart on the baking sheets. Use the palm of your hand to press the balls into 1-inch circles. Press 4 slivered almonds into each cookie, arranging them decoratively to form an X. Brush the tops of the cookies with the egg white.
6. Bake the cookies until golden and crisp around the edges, about 15 minutes. Cool completely on the baking sheets on a cooling rack. The cookies can be stored in an airtight container for up to 3 days.