2. Grease a 9-inch spring form pan and line the bottom of the pan with wax paper.
3. In a large bowl, cream the butter and sugar. Add the eggs, one at a time, beating well after each addition. Stir in the vanilla extract, then fold in the flour.
4. Transfer half of the batter to another bowl. Add the melted chocolate to one of the bowls and stir to blend.
5. Drop the batters alternately by large spoonfuls into the prepared pan, alternating between the plain and chocolate. Using the handle of a wooden spoon, make a single swirl through the batters to marbleize.
6. Bake for about 45 minutes or until the cake tests done when a wooden pick is inserted near the center and comes out clean. Let the cake cool in the pan for 5 minutes, then remove the cake from the pan and cool completely on a wire rack.