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Heat the oven to 180C/160C fan/gas 4. Butter and line a deep 20cm cake tin with baking parchment.
Mix the flour and cinnamon together in a large bowl. Add the butter and rub into the flour using your fingers, until it resembles fine breadcrumbs. Stir in the light brown sugar. Beat in the egg followed by 6-8 tbsp of milk – add it gradually until you have a smooth, thick batter.
Add the apples and sultanas and mix to combine. Scrape the batter into the prepared tin and gently level out. Sprinkle over the demerara sugar, if using, and bake for 30-40 mins or until golden and a skewer inserted into the middle comes out clean.
Allow to cool in the tin for 15 mins and then carefully turn out onto a wire rack to cool further. Best served still warm with a little custard.