Stand 12 silicone cake cases on a baking sheet, or line a 12-section bun tray with paper cases. Pare thin strips of rind from the lemon and one of the limes using a sharp knife. Slice the pared strips as finely as possible into shreds. Put them in a small saucepan and just cover with cold water. Cook very gently for about 15 minutes until the shreds are tender – they should be soft enough to break up when squeezed between your thumb and forefinger. Drain and leave to cool.
Squeeze the juice from the lemon and limes.
Put the butter, 150 g (5 oz) of the sugar, the eggs, flour, baking powder and ground almonds in a bowl. Beat with a hand-held electric whisk for about a minute until light and creamy. Divide the cake mixture between the silicone or paper cases.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 20 minutes or until risen and just firm to the touch. Transfer to a wire rack.
Scatter the rind over the cakes while still warm and drizzle with the juice. Sprinkle with the remaining sugar and leave to cool.