Preheat oven to 400°. In a large bowl, beat butter and 1/2 cup confectioners' sugar until blended. Beat in egg yolk and vanilla. In another bowl, whisk flour and baking powder; gradually stir into creamed mixture until blended (mixture will be crumbly). Shape heaping tablespoons of dough into crescents. Place 1 in. apart on ungreased baking sheets. Bake 12-15 minutes or until edges are lightly browned. Remove from pans to wire racks to cool 15 minutes.
Place remaining sugar in a small bowl. Toss slightly cooled cookies in sugar; return cookies to wire rack to cool completely. Toss cookies in sugar once more before serving. Store in an airtight container, adding any remaining confectioners' sugar to cover cookies.
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