Beat butter at medium speed of an electric mixer until creamy; gradually add sugar, beating well. Add egg and vanilla; beat well.
Combine flour, baking powder, and salt; stir well. Add flour mixture to butter mixture, beating at low speed.
Remove half of dough from bowl. Add orange rind and orange extract to dough in bowl, and mix well. Remove orange dough from mixing bowl, and set aside. Return plain dough to mixing bowl; add melted chocolate, beating well. Cover and chill both portions of dough 1 hour.
Roll each half of dough to a 15" x 8" rectangle on floured wax paper. Place orange dough on top of chocolate dough; peel off top wax paper. Tightly roll dough, jellyroll fashion, starting at short side and peeling wax paper from dough while rolling. Cover and chill 1 hour.
Slice dough into 1/4" slices; place on ungreased cookie sheets. Bake at 350° for 10 to 12 minutes. Remove to wire racks to cool.
Chocolate-Cherry Swirls: Substitute 1/2 cup drained minced maraschino cherries for orange rind and orange extract. Add 3 tablespoons all-purpose flour to cherry dough if it is too soft.
Note: To prevent flat-sided cookies, turn dough rolls halfway through the second chilling time. Dental floss makes cutting the dough easier.