1. Whisk flour, baking powder and salt in a small bowl. Beat butter, sugar, egg, egg yolk and vanilla in a large bowl until blended. Stir in flour mixture. Gather into a ball; wrap in plastic. Refrigerate 2 hours.
2. Heat oven to 350 degrees F. Roll out dough with a lightly floured rolling pin on a floured surface to 3/8 -inch thickness. Cut with 4-inch snowflake cookie cutter. Place on ungreased baking sheets, 1 1/2 inches apart. Bake at 350 degrees F for 10 to 12 minutes, until just browned at edges. Remove cookies to a wire rack to cool.Icing
3. In a medium-size bowl, combine confectioners' sugar, powdered egg whites and 2 tablespoons cool water. Beat on high with an electric mixer for 5 minutes, adding more water as needed for good piping consistency. Divide in half; transfer one half to a pastry bag fitted with a small writing tip. Thin remaining half with a little bit of water (to make a base coat). Spread some of the cookies with thinned icing (see sample above); let dry. Pipe decorations on cookies with thicker icing; sprinkle with sugar. Let icing dry 1 hour before stacking cookies.