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First, make the pastry. Tip the flour, butter, sugar and lemon zest into a food processor and blitz until the mixture resembles fine breadcrumbs. Tip in the egg yolk and ½-1 tbsp cold water and blitz again to bring everything together into a dough. If the dough isn’t coming together, add more cold water, ½ tsp at a time, and blitz again. Roll the pastry out on a lightly floured surface to a 3mm thickness, then use it to line the base of a loose-bottomed traybake or brownie tin (ours was 20 x 30cm). Transfer to the fridge and chill for 20 mins.
Heat the oven to 200C/180C fan/gas 6. Line the chilled pastry case with baking parchment and baking beans, then bake for 15 mins. Remove the parchment and beans and bake for 5 mins more until the pastry is lightly golden. Leave to cool for a few minutes before spreading over the lemon curd, leaving a 2cm border around the edges (it will spread out when the cake batter is poured over). Turn the oven down to 180C/160C fan/gas 4.
Beat the butter, sugar and lemon zest together using an electric whisk or in a stand mixer until pale, light and fluffy, about 8 mins. Beat in the eggs, one at a time along with 1 tbsp of the flour, then fold in the remaining flour and the ground almonds. Carefully spoon the batter over the lemon curd and gently spread out using the back of the spoon, then bake for 35-40 mins until a skewer inserted into the middle comes out clean. Leave to cool in the tin completely, then carefully remove to a board.
To make the icing, combine the icing sugar, lemon juice and elderflower cordial until loose enough to drizzle over the sponge, then swirl over extra lemon curd, if you like. Cut into squares and serve. Will keep in an airtight tin for up to three days.