Heat oven to 350°F. Grease and flour 13x9-inch pan. In large bowl, combine all cake ingredients; beat at low speed until moistened. Beat 2 minutes at high speed. Pour batter into greased and floured pan.
Bake at 350°F. for 35 to 40 minutes or until toothpick inserted in center comes out clean. Cool 1 hour or until completely cooled.
For frosting, in large bowl, beat butter until light and fluffy. Gradually add powdered sugar, beating well after each addition. Beat in orange marmalade. Add milk 1 tablespoon at a time, beating until of desired spreading consistency. Frost cooled cake.
In small saucepan, melt chocolate chips with oil, stirring constantly until smooth. Drizzle over frosting.