Blackberry crumble traybake

Method

1. Heat the oven to 180C/160°C fan/gas 4, then grease the cake tin with oil and line with baking paper. Make the streusel topping first. Stir together the 100g flour, oats, demerara sugar and lemon zest in a large bowl. Rub in the butter until you have lots of sticky clumps (don’t go as far as you would with a crumble). Set aside.
2. Combine the 140g flour, sugar, bicarbonate of soda and salt in a large mixing bowl. In a jug, whisk the oil and egg until combined, then whisk in the milk, vanilla and lemon zest. Pour the wet ingredients into the dry and stir to combine. Gently stir in the lemon juice. Tip the batter into the prepared tin.
3. Scatter half the streusel mixture evenly over the cake, then top with the blackberries. Finish with the remaining streusel mixture. Bake for 35-40 minutes until a skewer pushed into the centre comes out with only a few damp crumbs.
4. While the cake bakes, make the rosemary custard. Put the milk, cream and rosemary in a saucepan over a medium heat and warm through, stirring regularly to stop it catching. Meanwhile, in a large bowl, use a balloon whisk to beat the egg yolks and sugar together until combined and a little paler. Once the milk is steaming hot and quivering (but not quite boiling), gradually pour the milk mixture into the egg mixture, whisking constantly until everything is combined.
5. Pour the mixture back into the pan and put over a low-medium heat. Use a rubber spatula to gently stir the mixture as it heats through (you don’t want to add air bubbles at this point). Cook gently until the custard thickens enough to coat the spatula (about 5-7 minutes). It should reach 82°C when it’s ready but you can just check by dunking the spatula in the custard, then pushing your finger through it – if the line you’ve created in the custard holds for a few seconds it’s ready. Remove the rosemary and serve the custard hot or allow to cool.
6. Leave the cake to cool in the tin for about 10 minutes before transferring to a board to slice and serve warm with the hot custard. Or transfer to a wire rack and leave to cool completely.