To make the lemon sugar, whizz 50g golden caster sugar in a food processor with most of the lemon zest, then mix in the remaining sugar. Tip onto a baking tray and leave to dry for 1 hour.
To make the cakes, heat the oven to 180C/fan 160C/gas 4. Beat the sugar, butter and lemon zest using an electric mixer until light and fluffy - give it plenty of time as this will make the cakes nice and light. Add 1 egg at a time with a spoonful of the measured flour to prevent curdling. Finally, mix in the remaining flour and 2 tbsp lemon juice. Line a bun tin with 12 cases and divide the mix between them. Bake for 18-20 minutes, until risen and light golden. Cool on a wire rack.
Mix the icing sugar and 2 tbsp lemon juice until smooth. Remove the paper cases from the cakes and sit on wire racks. Ice the tops of the cakes. Sprinkle with lemon sugar and the remaining zest and leave to set.