1. Whisk together flour, baking powder, and salt in a medium bowl. Beat together butter and superfine sugar in a large bowl with an electric mixer until light and fluffy. Add whole eggs 1 at a time until well combined. Beat in vanilla, then on low speed slowly add flour mixture until just combined.
2. Turn dough out onto a lightly floured surface and knead until it just comes together. Quarter dough and form into disks. Wrap each in plastic and chill until firm, about 1 hour.
3. Preheat oven to 350 degree F and butter 4 large cookie sheets.
4. Roll 1 disk of dough on a lightly floured surface with floured rolling pin until 1/4 inch thick. Cut out dough with 5-inch snowflake cookie cutter, gather scraps of dough into a disk and re-chill until firm. Transfer cutouts to waxed paper-lined sheets and chill until firm, about 15 minutes. Brush tops of cookies with egg white and sprinkle with sanding sugar (this will keep excess sanding sugar off your baking sheets). Chill dough again if necessary. Carefully remove cookies from waxed paper to prepared cookie sheets and chill until firm, about 15 minutes. Repeat rolling, cutting, and chilling remaining dough and dough scraps in same manner.
5. Bake cookies, in batches, in upper and lower thirds of oven, switching position of sheets halfway through baking, until pale golden, 10 to 12 minutes. Transfer cookies on sheet to a wire rack to cool for about 2 minutes or until firm. Transfer cookies with a metal spatula to wire racks to cool completely.
6. Cookies will keep for 3 weeks stored between sheets of waxed paper in an airtight container. Makes about 4 dozen large snowflake cookies.