Rub butter into 300g plain flour with clean fingertips until it resembles breadcrumbs; mix in baking powder, 150g caster sugar, 2 egg yolks, salt and lemon juice until combined. Form dough into a ball; wrap in cling film and chill in fridge for 1 hour.
Warm half of the milk in a saucepan over low heat; whisk in 4 beaten eggs, sugar, flour and remaining milk until smooth. Add strip of lemon zest and cook over very low heat, stirring constantly, until thickened; take off the heat and remove strip of lemon zest.
Preheat oven to 180 C / Gas 4; lightly grease and line a 25cm tart tin with baking paper.
Roll out pastry on a lightly floured surface; lift onto the tart tin, pressing the pastry into the corners of the tin and trimming off any excess pastry. Prick the base with a fork and line with a circle of baking parchment; fill with baking beans and bake blind for 10 minutes. Remove the paper and beans and return the tart to the oven to cook for a further 5 minutes, or until golden brown.
Allow pastry case to cool completely; pour in custard, smoothing it out until level.
Spoon softened Nutella® into a piping bag with a thin nozzle; draw parallel strips across the top of the pie, about 1cm apart. Use a cocktail stick or wooden skewer to drag through the Nutella in opposite directions to create a feathered effect.