Grease a 13x9x2-inch baking pan; set prepared pan aside. Combine flour, baking powder, baking soda, and 1/4 teaspoon salt; set aside.
In a mixing bowl beat butter for 30 seconds. Add sugar and vanilla; beat until fluffy. Add eggs, one at a time, beating well after each. Add dry mixture and milk alternately to beaten mixture, beating on low speed after each addition just until combined.
Transfer 1-1/2 cups of the batter to another bowl; stir in chocolate-flavored syrup. Pour light batter into the prepared pan. Spoon chocolate batter atop. Gently cut through batters to marble.
Bake in a 350 degree F oven for 30 to 35 minutes or till a wooden toothpick comes out clean. Place cake in pan on a wire rack. Cool thoroughly. Makes 12 to 16 servings.