Preheat the oven to 180'C.
Scrape the seeds from the vanilla pod, cut the butter into small cubes and place in a bowl. Add the flour and the sugar and rub together with the tips of your fingers until the mixture resembles breadcrumbs. Add the egg and bring the mix together until it forms a ball. Knead with the palm of your hand until smooth. Roll the pastry out and line one large or six small tart case(s). Chill for 15 minutes. Line the pastry case with baking parchment and fill with baking beans. Transfer to the oven and blind bake for 15 minutes until golden brown. Remove the baking beans and return the pastry case to the oven for 5-10 minutes to finish cooking.
Take out and leave to cool.
Zest and juice the lemons, bring to the boil. Mix the corn starch with the water then add the sugar and egg yolks. Mix well and pour into the lemon juice, whisking until the mixture has thickened. Take off the heat and whisk in the butter a little at a time. Pour into a bowl and leave to cool.
Pour into the pastry cases once they have been blind baked and cooled.
Put the sugar and water into a saucepan and place over a high heat. Stir until the sugar has dissolved and bring the mixture up to 118c (soft ball stage). Meanwhile whisk the egg whites in a mixer until firm peaks are achieved. Reduce the speed and gradually pour in the sugar syrup. Turn the speed back up and continue whisking until the mixture has cooled down to room temp (approximately 5 minutes).
Spread or pipe the merinugue on top of the curd filling and brown with a blowtorch.
«Vary the flavours in the tart by replacing the lemons with limes, oranges or grapefruit.»