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Tip the flour, cornmeal, butter, sugar and 1 tsp salt into a large bowl and rub together until the mixture resembles breadcrumbs. Add 50-100ml water, a little at a time, until a dough forms.
Butter a loose-bottomed 23cm tart tin. Roll the pastry out on a lightly floured surface until it’s just larger than the diameter of the tin and around 2mm thick. Loosely drape the pastry over the rolling pin and unravel into the tin. Carefully push the pastry into the base and up the side. Use a fork to prick the base all over, then chill for 30 mins.
Heat the oven to 180C/160C fan/ gas 4. Scrunch up a sheet of baking parchment and use it to line the pastry case, then fill with baking beans or uncooked rice, gently pushing these down. Bake for 15 mins, then remove the beans and parchment and bake for 10 mins more until lightly golden. Remove from the oven and leave to cool.
While the pastry case is baking, make the custard. Tip the sweetcorn into a heavy-based, wide pan and cook over a medium-high heat for 6 mins until the corn smells toasted. Add the butter and reduce the heat to medium. After about 5 mins, or when the butter starts to smell nutty, add the 3 tbsp of sugar and continue to stir. After a couple more minutes, stir in the milk and turn off the heat. Set aside to cool slightly. Whisk the egg yolks along with the remaining 75g sugar in a medium bowl until the mixture is pale, yellow and creamy.
Transfer the corn mixture to a food processor and blitz until smooth. Push through a sieve into a large bowl and discard the fibrous outer kernels caught in the sieve. (You may need to do this in two batches.)
While the sweetcorn mixture is still hot, slowly add it to the egg yolk mixture, whisking continuously as you do. Stir in the vanilla and crème fraîche until just combined – be careful not to over-mix, as air bubbles may form on the surface.
Pour the custard into the cooled pastry case, then bake for 25-35 mins until the mixture is set with a slight wobble in the middle. Generously grate over some nutmeg, then leave to cool completely before slicing. Serve with extra crème fraîche, if you like.