Sift the flours, baking powder, bicarbonate of soda and salt into a large bowl, then add the wheatgerm left in the sieve. Stir in the caster sugar. Mix together the bananas, egg, measured water and oil in a jug, then pour into the dry ingredients and gently mix until just combined. Fold in the carob or chocolate.
Line a 12-hole muffin tin with 12 paper muffin cases and three-quarters fill each with the mixture.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 20–25 minutes until they are well risen and spring back when you press them. Place on a wire rack to cool.