In a large mixing bowl, beat the butter with an electric mixer on medium high speed for 30 seconds. Add the granulated sugar, brown sugar, and baking powder; beat until combined. Beat in the egg and vanilla. Beat in as much of the flour as you can with the mixer. Stir in any remaining flour with a wooden spoon. Divide the dough in half.
Color half the dough red and the other half green. Shape red and green halves of dough into 8-inch logs. Wrap each in waxed paper or plastic wrap. Chill for 2 hours, or until firm. Cut each chilled log lengthwise into quarters; reassemble logs, alternating red and green quarters. Wrap and refrigerate for 15-30 minutes, or until well chilled.
Cut dough into ¼ inch thick slices. Place 2 inches apart on parchment lined baking sheets. Flatten slightly with the palm of your hand. Bake in a preheated 375°F oven for 8-10 minutes, or until edges are firm and bottoms are lightly browned. Remove cookies and cool on a rack.