Blitz the digestive biscuits to a fine crumb in the food processor and tip into a bowl.
Add the melted butter and stir to distribute the butter evenly.
Tip the crumb into a greased 23cm (9in) springform tin. Press down firmly to create an even base.
Bake for 5 minutes only, then set aside.
Make the blackberry cheesecake filling
Put the chopped blackberries in a bowl and add the tablespoon of flour. Mix together.
Put the cream cheese, double cream, vanilla, eggs and egg yolk, caster sugar and flour in a bowl.
Whisk just enough to combine - don't whisk too vigorously as you don't want to introduce excess air bubbles.
Tip in the floured blackberries. Lightly fold together, leaving the mix streaky.
Pour into your cheesecake tin, on top of the biscuit base.
Top with the blackberries halves.
Bake
Bake for 1 hour or until set but still jiggly if gently shaken.
As soon as it comes out of the oven, run a knife around the inside of the tin. This stops the cheesecake from sticking to the sides and cracking as a it cools.
Allow to cool to room temperature, then cover and chill in the refrigerator overnight.
Serve
When you're ready to serve, run a knife around the inside of the tin and unlock to release the sides. It should then lift onto a board cleanly.
Slice with a sharp knife to serve, wiping between cuts for a neat finish.