1 lemon, zest only
1 tbsp custard powder or cornflour
4 tbsp lemon curd
280g/10oz plain flour
85g/3oz caster sugar
130²µ/4ΒΎ΄Η³ϊ icing sugar
150g/5Β½oz salted butter, softened
75g/2ΒΎoz unsalted butter, softened
1 large free-range egg yolk