For cinnamon crème anglaise, bring cream, milk and cinnamon quills just to the simmer over low heat. Meanwhile, whisk yolks, sugar and ground cinnamon in a bowl until pale (2-3 minutes), pour cream mixture over and whisk to combine. Return mixture to pan and stir over medium heat until mixture coats the back of a wooden spoon thickly (4-5 minutes), strain through a coarse sieve and refrigerate until chilled. Makes about 500ml.
Preheat oven to 220C and place an oven tray in oven while it’s heating. Trim pastry to a 23cm x 26cm rectangle, place on a piece of baking paper and prick all over with a fork.
Thinly slice apples on a mandolin to 2mm thick and arrange over pastry, overlapping slices and leaving a 5mm border. Combine sugar and cinnamon in a bowl, scatter half over apples. Dot apples with half the butter, then transfer pastry and baking paper to heated oven tray and bake on top shelf of oven until pastry begins to brown (8-10 minutes). Scatter with remaining cinnamon sugar, dot with remaining butter and bake until apples are tender and browned on edges and pastry is dark golden and crisp (10-15 minutes). Cool slightly on tray (5 minutes), then transfer to a wire rack. Dust edges with icing sugar and serve warm with cinnamon crème anglaise.