- Preheat the oven to 190°C. Line a shallow 20cm (8in) square cake tin or brownie tin with baking parchment.
- Cream the butter and sugar together until mousse-like and doubled in volume.
- Add the eggs, one at a time, beating between additions. Add the vanilla extract and lemon zest.
- Mix in the flour and baking powder.
- Toss the apple pieces in the lemon juice.
- Add half the apple to the cake batter and mix in.
- Gently mix in half the blackberries. Pour into the prepared cake tin.
- Dot the rest of the apple pieces and the remaining blackberries on top of the cake batter and sprinkle the demerara all over.
- Bake for 30-35 minutes until golden and the centre is firm to touch.
- Leave in the tin for 10 minutes before turning out and allowing to cool completely. Cut into squares.
For the custard
- Put the milk and cream in a saucepan and heat until just before it begins to simmer – you will see tiny bubbles at the side of the pan and steam will start to come off the milk.
- Whisk the yolks, sugar and cornflour together in a bowl until well blended.
- Pour the hot milk and cream on to the eggs and sugar, whisking all the time with a balloon whisk.
- Return, through a sieve, back into the pan, add the vanilla and heat gently until it’s thick enough that a line drawn with your finger onto the back of a wooden spoon stays there.
- Pour the custard into a jug and serve at once or keep in the fridge covered with cling film and gently reheat to serve.