Beat butter at medium speed with an electric mixer until creamy. Gradually add 1 cup sugar, beating until smooth. Add egg and vanilla, beating until blended.
Combine flour, baking powder, and salt; gradually add to butter mixture, beating just until blended. Shape dough into a ball; cover and chill 2 hours.
Divide dough into 3 portions. Work with 1 portion of dough at a time, storing remaining dough in refrigerator. Shape dough into 1 1/2" balls; roll each ball in decorator sugar. Place 2" apart on parchment-lined baking sheets. Gently press and flatten each ball of dough to 3/4" thickness. Bake at 375° 13 to 15 minutes or until edges of cookies are lightly browned. Cool 5 minutes on baking sheets; remove to wire racks to cool.