Put the butter and sugar into a large bowl and use an electric mixer to cream together for 5 minutes or until light and fluffy. Add the eggs a little at a time, mixing until combined before adding more. Meanwhile, mix together the flour, baking powder and 1 /4 tsp of salt in a large bowl. Once all the eggs are combined, add the flour mixture in three additions, alternating with the soured cream and starting and finishing with the flour. Divide the batter equally into two bowls. Sift the cocoa powder over one portion, add the milk and beat together until smooth and combined.