Place the butter, sugar and vanilla in a stand mixer with the paddle attachment and beat for 4-5 minutes until pale and fluffy. Add the eggs, one at a time, beating well after each addition. In a separate bowl, sift together flours, cocoa and bicarb soda. Combine buttermilk and milk in another small bowl or jug. In batches, fold flour mixture and milk mixture, alternating between the two, into the butter mixture until just combined and smooth.