½ tsp lemon juice
230ml/8fl oz condensed milk
170ml/6fl oz evaporated milk
150²µ/5½´Ç³ú plain flour
2 tbsp icing sugar
175g/6oz light muscovado sugar
75g/2½oz unsalted butter, chilled, cut into roughly 1cm/½in cubes
1 free-range egg yolk