Line 2 x 12-section mini muffin tins with paper cases.
Place the flour, butter, caster sugar, baking powder, banana and eggs in a bowl and beat with a hand-held electric whisk until smooth. Stir in the toffees. Divide among the cases and sprinkle over most of the muscovado sugar.
Bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 10–12 minutes until just firm. Remove the cakes from the oven and transfer to a wire rack to cool.
Top with chewy banana slices or banana chips and sprinkle with the remaining sugar.