First make your crumble by mixing the sugar, butter and cinnamon in a large bowl. Add in the flour and mix with a spoon until you have a crumbly mixture. Set aside.
Preheat the oven to 180c/350f. Line a 24cmx24cm (9 1/2 inch x 9 1/2 inch) square baking tin with baking parchment. Note - the sides of the tin need to be at least 8cm/3.1 inches high as the cake will spill over if the sides aren't high enough.
Peel and core the apples and chop into bite-sized chunks. Mix the apples with the lemon juice to stop them turning brown. Then mix with the 2 tbsp of flour.
In a large bowl, mix the melted butter with the caster sugar, vanilla extract and cinnamon. Mix in the eggs and then fold in the flour, baking powder and salt. Gently stir the apples into the cake mixture.
Spoon the mixture into the prepared baking tin.
Sprinkle the crumble mixture over the top (I like to have a mix of small and large chunks) and then sprinkle over the demerara sugar.
Place in the oven and cook for 45-55 minutes - until an inserted skewer comes out clean. Cover with foil for the last 10-15 minutes if it's starting to look too brown.
Take out of the oven and leave to cool in the tray for 10 minutes, then remove from the tray and leave to cool completely on a cooling rack.
Once cool, whisk together the icing sugar, half of the cream and the vanilla extract. If it looks drizzle-able stop there. If not, continue to add more cream until you get a thick but pourable mixture. Drizzle over the top of the cake using a spoon, or you can use a piping bag (like I did) to get the neat lines of drizzle.
Cut into 16 pieces and serve.
Can I make it ahead?
Yes, this cake should last 3-4 days in a sealed container at room temperature.You can also freeze the cake by wrapping in clingfilm/plastic wrap, then foil. Defrost at room temperature. The crumb topping won't be as crispy after freezing, but you can warm the cake through in the oven at 160C/320F for 20-25 minutes (until warm in the centre) and this should help the topping become crispy again..
Can I replace the apples with a different fruit?
Yes, but it's best to replace the fruit with one that isn't too juicy. Plums or pears are a great option..
Can I make it gluten free?
I haven't tried making this gluten free yet, but if I did, I would use my gluten-free White Chocolate and Pistachio Blondies recipe as a base (adding the apples into the mixture). Then i'd top with the crumble mixture - but would replace the flour with half gluten-free plain/all-purpose flour, quarter ground almonds and quarter gluten free oats. .Nutritional Information is per serving.