In a food processor, process the 5 oz. hazelnuts with 1/2 cup of the flour until fine-textured but not powdered. Add the remaining flour, sugar, baking powder, cinnamon, and salt. Pulse to combine. With your fingers, toss the butter in the flour to coat it, being careful to avoid the processor’s blade. Pulse until the mixture looks like cornmeal. Add the 2 egg yolks (reserve the whites), the vanilla, and the zest and pulse until the dough just begins to hold together. Cover the dough with plastic wrap and refrigerate until firm.
Heat the oven to 350ºF. Line two baking sheets with parchment. Lightly beat the reserved egg whites. Shape the dough into 1-inch balls. Roll each ball in the beaten egg whites and then in the chopped 3-1/2 oz. hazelnuts. (flour your hands lightly if necessary). Arrange the balls 1-1/2 inches apart on the lined baking sheets. Flatten them slightly with your palm. Press your thumb (floured if necessary) into the center of each cookie to create an indentation. If the dough splits apart, gently press it back together. Fill each cookie with about 1/2 tsp. of the preserves.
Bake the cookies until lightly browned, about 20 minutes. Let them cool on the baking sheets on a rack. When cool, lightly sift the confectioners' sugar over the cookies. Dip your finger in water and tap the center of each cookie so the jam shines through the sugar.
nutrition information (per serving):
Photo: Scott Phillips