Cream butter; gradually add sugar, beating well. Add egg and vanilla; mix well.
Sift together flour, baking powder, and salt; gradually add to creamed mixture, beating well after each addition.
Divide dough in half; wrap in waxed paper, and chill at least 2 hours. Roll half of dough to 1/4- inch thickness on a lightly floured surface; keep remaining dough chilled. Cut dough with a bell-shaped cookie cutter. Place on greased baking sheets; sprinkle with decorator sugar crystals.
Bake at 350° for 10 minutes or until edges are lightly browned. Cool on a wire rack. Repeat procedure with remaining dough.