In a small bowl, mix together the all-purpose flour, hazelnut flour and salt. Beat the butter and granulated sugar with an electric mixer on medium-high speed until creamy. Split the vanilla bean, scrape out the seeds using the back of a thin knife and add to the butter with the egg yolk. Beat until combined. Gradually mix in the flour mixture until just combined.
Roll the batter into a 12-inch log, wrap in plastic wrap and chill for 1 hour.
Preheat the oven to 350 degrees F. Line 2 baking sheets with parchment paper.
Cut the log into 18 pieces and roll each piece into a crescent shape. Put the crescents on the lined baking sheets, about 1 inch apart. Bake until the cookies are set and the bottoms and edges are browned, about 14 minutes. Cool on the baking sheets for a few minutes. Dust with confectioners' sugar while warm, and then transfer to a wire rack to cool completely.
Recipe courtesy of Geoffrey Zakarian