In a large mixing bowl, beat butter with an electric mixer on medium to high speed 30 seconds. Add sugar, baking powder and salt. Beat until combined, scrapping sides of bowl occasionally. Beat in egg and vanilla until combined. Beat in as much of the flour as you can with the mixer. Using a wooden spoon, stir in any remaining flour.
Divide dough in half. Stir orange peel into one dough half; stir melted chocolate into the other. If necessary, cover and chill about 1 hour or until dough is easy to handle.
Shape each half of dough into a 10-inch-long log. Wrap each roll in plastic wrap or waxed paper. Chill for 2 hours or until firm.
Cut each chilled roll lengthwise into quarters; reassemble rolls, alternating chocolate and orange quarters. Wrap and chill for 30 minutes or until firm.
Preheat oven to 375 degrees.
Using a shape knife, cut dough into 1/2-inch slices. Place slices 1 inch apart on ungreased cookie sheet.
Bake for 7 to 9 minutes or until edges are firm and bottoms are lightly browned. Transfer cookies to a wire rack; cool.
Makes about 5 dozen.
Source: Cookies for Christmas by Jennifer Dorland Darling (Meredith Books, 2001).