Beat butter at medium speed with an electric mixer 2 minutes or until creamy. Gradually add sugar, beating 5 to 7 minutes. Add eggs, 1 at a time, beating just until yellow disappears.
Add flour to butter mixture alternately with milk, beginning and ending with flour. Mix at low speed just until blended after each addition. Stir in vanilla.
Combine 1 cup batter and chocolate syrup, stirring until blended.
Divide remaining batter in half; pour 1 portion into a greased and floured 10-inch tube pan. Spoon half of chocolate batter on top; repeat layers. Gently swirl batter with a knife.
Bake at 300° for 1 hour and 40 minutes or until a wooden pick inserted in center comes out clean. Cool in pan on a wire rack 10 minutes; remove from pan, and cool completely on a wire rack.