Preheat oven to 350ºF. In large bowl, with mixer at low speed, beat butter and sugar until blended. Increase speed to high, beat until light and creamy. At low speed, beat in egg and vanilla. Beat in flour and baking powder just until blended. Divide dough into 4 equal pieces. Wrap each piece with plastic wrap and refrigerate until firm enough to roll, about 1 hour. On lightly floured surface, with floured rolling pin, roll 1 piece of dough 1/8 inch thick. With floured 2- to 3-inch assorted cookie cutters, cut dough into as many cookies as possible, wrap and refrigerate trimmings. Place cookies, 1 inch apart, on large ungreased cookie sheet, sprinkle cookies with colored sugar now if you like, or frost with Ornamental Frosting after baking. Bake cookies 10 to 12 minutes, until lightly browned. Transfer cookies to wire rack to cool. If you like, brush colored sugar remaining on cookie sheets onto piece of waxed paper to use again. Repeat with remaining dough and trimmings. When cookies are cool, prepare Ornamental Frosting if you like, use to decorate cookies as desired. Sprinkle colored sugars as desired on frosting before it dries. Allow frosting to dry completely, about 1 hour. Store cookies in tightly covered container up to 2 weeks.