Preheat oven to 160C (gas mark 3).
Prepare a 9-inch springform tin by rubbing the base and edges with butter.
Beat up 200g of plain digestive biscuits inside a sandwich bag or bowl and combine with 100g of melted butter.
Push the mixture down inside the 9-inch springform cake tin and then I bake for around 6 minutes to firm up a little.
Melt the plain chocolate in a bowl over a saucepan of boiling water. Alternatively microwave for 10 seconds at a time, taking care not to burn it.
Separate the mixture into two bowls. Place 2/5 of the mixture in one bowl and 3/5 of the mixture in the second bowl.
Add the chocolate to the bowl with 2/5 of the mixture. Combine this.
Add spoonfuls of the plain mixture and the chocolate mixture into the Springform tin. Alternate to create a polka dot pattern.
Once you've dispensed all of the mixture then use a chopstick, knife or similar implement to create swirls.
Return the tin back to the oven, at 160c (gas mark 3) for 30-35 minutes.
The cheesecake is ready when the edges look slightly puffy and there is only a small wiggle left to the cheesecake.
Turn off the oven and leave the cheesecake to cool fully.