Preheat the oven to 350º F. Prepare two 8-inch cake pans and two 6-inch cake pans.
Melt the chocolate in the microwave in 30-second intervals until it is smooth. Set the chocolate aside.
In a separate mixing bowl, cream the butter and oil with an electric mixer until smooth. Add the sugar and mix until light and fluffy.
Add the eggs, one at a time. Mix in the vanilla extract and beat the mixture until it is smooth and well combined.
Mix in the flour, baking powder, and salt in multiple portions, alternating with the buttermilk. Beat the mixture until it is smooth.
Pour a little less than half the batter into another mixing bowl. Pour in the melted chocolate and mix until it is well combined.
Scoop the batter into the prepared cake pans, alternating between chocolate and vanilla, swirling in between. The batter should be divided evenly.
Bake the 6-inch cakes at 350 degrees for 30-40 minutes and the 8-inch cakes for 40-50 minutes or until a skewer comes out clean when inserted. Remove the cakes from the oven.
Let the cakes cool in the pans on a cooling rack for 5 minutes. Remove the cakes from the pans to cool completely.