Rub together or stick in the food processor the flour, icing sugar, 125 grams butter and a pinch of salt until the mixture resembles fine breadcrumbs. Add 1 yolk and 2-3 tablespoons iced water, and continue until mixture comes together in a smooth ball. Cover and chill for 30 minutes.
Meanwhile, place cornflour in a bowl with the water and stir until smooth. Place in a pan with lemon juice and the 110 grams caster sugar, and stir over low heat for 5-6 minutes until thickened. Remove from heat and add remaining yolks, one at a time, combining well with a wooden spoon. Stir in limoncello and remaining 65 grams butter, then cover surface with a piece of cling film (to prevent a skin from forming) and cool.
Lightly grease 4-6 tart tins. Roll out pastry and fit in the tins. Chill in the fridge for 15 minutes.
Preheat oven to 180°C. Bake for 15 minutes. Pour lemon filling into the pastry cases and chill for 1 hour.
Whisk egg whites until soft peaks form. Gradually add remaining sugar and beat until glossy and firm. Pipe or spoon onto tarts and either stick under the grill or torch to get toasty!
