Heat the oven to 180C/fan 160C/gas 4, and butter 3x18cm cake tins, then line their bases with circles of baking paper. Beat the butter, peanut butter and sugar together with 1 egg until pale and light. Mix the mashed banana, remaining eggs, yogurt and vanilla together, then pour into the peanut butter mixture followed by the flour, baking powder and spices. Combine to a smooth batter, then divide equally between the tins – weighing the mixture will give you even layers. Bake on the middle shelf for 25-30 minutes – a skewer poked into the centre should come out clean when the cakes are done. Cool in the tins for 10 minutes, then turn out onto wire racks to cool completely.