1 lemon, zest only
1 tbsp custard powder or cornflour
4 tbsp lemon curd
280g/10oz plain flour
85g/3oz caster sugar
130驳/4戮辞锄 icing sugar
150g/5陆oz salted butter, softened
75g/2戮oz unsalted butter, softened
1 large free-range egg yolk