Beat the butter and sugar together in an electric mixer until fluffy. Mix in the zest and vanilla and, with the machine running at medium speed, beat in the eggs a little at a time. The mixture will look curdled but don’t worry – it will come together when you add the flour.
Mash the bananas, getting rid of as many lumps as possible, and stir into the butter and egg mixture, followed by the sour cream. Using a large metal spoon, fold in the flours with the baking powder and bicarbonate of soda and the nuts and seeds. Be careful not to over stir the mixture.
Spoon into a buttered and base-lined loaf tin measuring 23 x 13 cm and scatter the rest of the nuts and seeds over the top. Bake in an oven preheated to 180°C for an hour. A skewer inserted into the middle should come out clean if the banana cake is cooked.
Leave the cake in the tin to cool for 5 minutes, then run a knife between the sides of the cake and the tin to loosen. Turn it out onto a wire rack. Peel off the paper, put the cake the right side up and leave to cool completely. You can sift a light dusting of icing sugar over the top, or just leave it as it is. This banana cake is a good keeper – wrapped in foil it will be good for four days.