Heat oven to 180C/160C fan/gas 4. Lightly grease 2 x 23cm loose-bottom cake tins and line with a circle of baking parchment. Beat together the butter and sugar until pale and creamy. Add the eggs, one at a time, beating well between each addition, then add the baking powder, cocoa and a third of the flour. Mix well to combine, then stir in half the soured cream. Alternating between adding the flour and soured cream, mix everything until just combined. Spoon into the prepared tins, then smooth over the surface with a spoon.