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Pour the milk and cream into a saucepan over a low heat and warm until just simmering, then remove from the heat. Meanwhile, tip the egg yolks into a bowl, add the sugar and custard powder, and whisk together until smooth. Pour over the warm cream mixture, stir well, then pour back into the pan. Cook over a medium heat for about 5 mins, stirring constantly, until you have a thick custard. Cover the surface with baking parchment or wax wrap so it doesn’t form a skin. Will keep covered and chilled for up to three days.
To assemble the pie, mix the icing sugar and cinnamon together in a bowl and set aside. Brush a 22cm loose-bottomed cake tin with some of the butter. Unravel the filo pastry and drape a sheet into the cake tin with some overhang. Brush with more butter, and dust with a little of the cinnamon sugar. Repeat with another sheet of pastry, turning it by about 45 degrees over the first, then do the same again with the last three pastry sheets, always brushing with butter and dusting with the sugar. Spoon in and smooth over the custard filling, then pull the overhanging pastry up and towards the centre, scrunching it to enclose the filling. Drizzle with any remaining butter and dust with all but about 1 tbsp of the sugar mix.
Heat the oven to 180C/160C fan/gas 4. Put the tin on a baking tray and bake for 30-35 mins until the pastry is crisp and golden brown. Leave to cool a little, then carefully remove from the tin and transfer to a serving plate. Leave to cool completely to room temperature. Will keep for up to a day, covered, at room temperature. Dust with a little icing sugar and cut into wedges to serve.