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Prepare pancakes following the classic recipe (see 'Goes well with').
Beat together the mascarpone, vanilla extract, 1 tbsp of the sugar and the egg yolk until smooth and thoroughly combined, set aside. Melt half the butter in a large frying pan and add half the apples and half the remaining sugar. Cook until the apples are tender and starting to caramelise. Remove from the pan and cook remaining apples in the same way. Set aside. Add the blueberries to the pan, cook for 1-2 mins until they start to soften, remove from the pan and add half to the apples.
Place all the ingredients for the sauce in a saucepan with 75ml of water and cook over a low heat until the blueberries start to break down. Push the mixture through a fine nylon sieve, taste and add sugar or lemon juice if needed. Add the reserved softened blueberries and pour into a jug for serving.
Heat the oven to 180C/160C fan/gas 4. Lay the pancakes out on a work surface and divide the mascarpone mixture into the middle of each pancake in a neat dollop. Top the mascarpone with the apple and blueberry mixture and fold over the sides of the pancakes to completely encase the filling in a neat square parcel. Arrange the blintzes in an ovenproof dish seam side down and scatter with a little caster sugar plus a pinch of ground cinnamon.
Bake on the middle shelf for about 10-15 mins or until warmed through. Serve 2 blintzes per person with the blueberry sauce alongside in a jug.