Put the almond flour, butter, and salt into a second bowl and mix until the mixture resembles cornmeal, about 3 minutes if using food procesor with paddle attachment on medium speed.
Add the egg and almond extract and mix until well-incorporated.
Add the flour mixture. Mix just until no traces of flour remain.
Transfer the dough to a large sheet of plastic wrap, flatten into a 1-inch thick disc, cover tightly, and refrigerate until hard, at least 30 minutes.
Preheat the oven to 325°F and line 2 baking sheets with parchment paper.
Form the dough into 1/2-inch balls and put on the baking sheets 1 inch apart. Use the palm of your hand to press the balls into 1-inch circles. Press slivered almonds into each cookie, decoratively arranging them.
Brush the tops of the cookies with the egg white, then bake until golden and crisp around the edges, 15 minutes.
Cool completely on the cookie sheet and serve or store in an airtight container for up to 3 days.