Beat the remaining 2 tablespoons butter, oil and granulated sugar in a large bowl with an electric mixer until light and fluffy, about 2 minutes. Beat in eggs, one at a time. Mix in vanilla and the zest. With the mixer on low, blend in the flour mixture alternately with buttermilk, making 3 additions of the flour mixture and 2 of the buttermilk and mixing until just combined. Pour the batter into the pan, smoothing the top.