Line a 12-section bun tray with paper cake cases. Put the butter and bran flakes in a heatproof bowl.
Bring the milk almost to the boil in a saucepan and pour into the bowl. Leave to stand for 10–15 minutes until the bran flakes are very soft and the mixture has cooled slightly, then stir in the agave nectar or sugar and raisins.
Sift the flour and baking powder into the bowl and then stir until just mixed. Divide the cake mixture between the paper cases.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 20 minutes or until slightly risen and just firm to the touch. Transfer to a wire rack to cool.