Preheat oven to 350°F. Grease and flour two 9-inch round cake pans.

In a large bowl, beat butter and sugar until light and fluffy. Add eggs one at a time, mixing well after each addition.

Gradually add flour, alternating with buttermilk, beginning and ending with flour. Stir in vanilla.

Divide batter equally into two bowls. Mix cocoa powder into one bowl for the chocolate layer.

Pour batter into prepared pans, alternating colors to create striped effect. Bake for 25-30 minutes or until a toothpick inserted in the center comes out clean.

Allow cakes to cool completely before assembling and frosting.