Cream butter; gradually add sugar, beating well. Add eggs, one at a time, beating mixture well alter each addition.
Combine flour, baking powder, and salt; add to creamed mixture alternately with milk, beginning and ending with flour mixture.
Divide batter in half; add remaining ingredients to one half of batter, stirring until smooth. Drop batter by tablespoonfuls into a greased 9- x 5- x 3-inch loafpan, alternating dark and light mixtures. Bake at 350° for 1 hour or until a wooden pick inserted in center comes out clean.