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Heat the oven to 200C/180C fan/gas 6. Roll out 250g of the pastry on a sheet of baking parchment to about 25cm square. Scatter over half the almonds, then press them in very gently, dust with half the icing sugar and lay over a second sheet of baking parchment. Lift onto a baking sheet, and sit another baking sheet on top like a sandwich. Fill a few small tins with baking beans (or something similarly heavy and ovenproof, like uncooked rice). Bake for 30 mins, then have a quick peek – it will probably need another 5 mins to be well browned and crisp all over.
Remove from the oven, take off the weights and top baking tray, then carefully lift off the top sheet of parchment and leave to cool. Repeat with the remaining pastry – you can do both sheets at once if you have enough baking sheets and tins, but they might not cook as evenly with two in the oven, so it is safer to do one at a time. Cool both completely.
Put the gelatine in a bowl of cold water to soften, then put the remaining custard ingredients in a big saucepan. Set over a low heat to start, whisking continuously to mix well, then increase the heat a little until the custard is gently boiling. Keep whisking the whole time – the custard will thicken and lump unevenly, but keep cooking and whisking until it is smooth and very thick.
Remove the custard from the heat and take out the vanilla pod. Squeeze out the water from the gelatine sheet, then stir it into the custard until it has dissolved.
Line the base and sides of a 20cm square tin with baking parchment – criss-crossing two big strips will give you the neatest corners. Using a big, sharp knife and a ruler – or the base of your tin if it is removable – trim one pastry sheet to a shade under a 20cm square, so it will sit inside the tin, almond-side down. Spread over the custard as evenly as you can. Trim the second pastry sheet in the same way, but then divide the square into eight 10 x 5cm rectangles (so it will be easier when you cut the slices later). Sit them on top of the custard, almond-side up, putting them back together a bit like a jigsaw puzzle. Gently press down on the pastry so it doesn’t break but sticks to the custard, then cover and chill for 4 hrs or so, but better still overnight.
To serve, lift the slice from the tin and cut all the way through where the pastry top is already cut. Dust the slices with a little more icing sugar and serve. Best eaten within 24 hours, when the pastry is still good and crisp.