Preheat the oven to 350 degrees F. Toast the walnuts until lightly browned, 12 to 15 minutes. Let cool, then finely grind them in a food processor. Sift flour and salt into a bowl and set aside. In the bowl of a stand mixer fitted with a paddle attachment on low speed, beat together the butter and sugar until lightened, about 4 minutes. Add the yolk and mix until fully incorporated. Add the dry ingredients and walnuts and mix just to combine. If the dough is soft, wrap it tightly in plastic and refrigerate for 30 minutes. Otherwise, roll out the dough about 1/2 inch thick on a lightly floured work surface into a 6-by-7-inch rectangle. Using a ruler, square the edges as much as possible. Using a sharp knife, cut the dough into1-inch squares. Place them about 2 inches apart on a parchment-lined baking sheet. Sprinkle lightly with sea salt. Preheat the oven to 350 degrees F. Bake the cookies until they are firm, about 10 to 12 minutes. Remove to a wire rack to cool. Store in airtight containers for up to 2 weeks.
Reprinted with permission from Miette by Meg Ray with Leslie Jonath, Chronicle Books copyright (c) 2011