Preheat the oven to 180°C, fan 160°C, gas 4. Butter one silicone swirl 6-cup mould or line a cupcake tin with 12 paper cupcake cases.
Zest 4 clementines or easy peelers and squeeze the juice of 1. Reserve a little of the zest for decoration.
Place the butter and caster sugar in a large mixing bowl and beat until light and creamy – about 5-7 minutes by hand or 3 minutes with an electric whisk. Gradually add the eggs, beating well after each addition; add a little flour if the mixture curdles. Fold in the rest of the flour, and the clementine zest.
Divide half the mixture between the silicone moulds (or fill all 12 cupcake cases) and bake for 20-25 minutes until golden brown and just firm to the touch. Repeat with the remaining mixture if using the silicone moulds. Allow to cool for 5 minutes then turn out of the mould and put on a wire rack with baking paper underneath.
Sift the icing sugar into a bowl. Add 5-6 teaspoons of the clementine juice to the icing sugar and beat until it makes a thin, runny icing. Pour over the warm cakes and allow to set. Top with the reserved zest.