Heat oven to 325 degrees F. Line a baking sheet with a silpat (a French nonstick baking mat) or parchment paper. In the bowl of a food processor fitted with the metal blade, combine the confectioners' sugar and the almonds. Process until finely ground. Add flour and salt and pulse to combine.
Scrape the seeds from the pod of vanilla bean. Add to the flour mixture. (Reserve the bean for the dusting sugar.) Add the butter a few pieces at a time. Quickly pulse to combine. Do not over process. The mixture should resemble coarse meal.
Place the reserved vanilla bean on a silpat or parchment paper lined baking sheet and let dry out in the oven for 3 to 5 minutes.
Remove the dough to a clean work surface. Blend the dough by pushing it away from you with the heel of your hand. Gather it up with a bench knife and continue with this process, until the dough will peel easily from the work surface in 1 piece.
Divide dough into 4 equal pieces. Roll each piece into a cylinder 5/8-inch in diameter and about 22 inches long. Cut each log into pieces, about 3 inches long. Curve each piece into a horseshoe, and place on the prepared baking sheet about 1 1/2 inches apart. Bake until lightly golden, 15 to 20 minutes.
While the cookies are baking, cut the reserved dried vanilla bean into small pieces. Place in the bowl of the food processor, fitted with the metal blade, with the remaining cup of confectioners' sugar. Process until well-mixed, about 1 minute. Sift the sugar into a small mixing bowl to remove any large pieces of the vanilla bean.
Remove the cookies to a rack to cool. After the cookies have cooled 5 minutes gently toss warm cookies in vanilla sugar, to coat. Remove to a rack to cool completely. Once cooled, dust again with remaining powdered sugar. Store in an airtight container for up to 1 week.
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