Preheat oven to 180°C (fan) and grease a 23cm tart tin. Roll out pastry, press into tin, trim edges, and prick base. Line with baking paper and fill with baking beans.
Blind bake for 10–12 minutes, remove beans and paper, then bake 5 more minutes until golden. Let cool completely.
Melt butter and sugar in a saucepan over medium heat, stirring until smooth and glossy.
Mix cornflour with a splash of cold milk. Add remaining milk to the pan, then whisk in cornflour paste. Cook, stirring, until thickened (about 4–6 minutes).
Once thick, remove from heat. Whisk in egg, vanilla, and lemon juice until smooth.
Pour filling into tart shell, smooth the top, and cool. Chill in the fridge for 2–3 hours until set.