Chocolate zebra cheesecake (Baked)

Method adapted from here: http://www.notquitenigella.com/2010/04/27/zebra-cheesecake/
The method remains largely the same as the usual cheese cake but the difference if pouring it into the pan. Unfortunately my lines aren't as regular as hers and I couldn't remove the bubbles because I couldn't use a spatula to slash the batter. It's a very messy job and towards the end, I got quite bored!

Ingredients
250g cream cheese, room temp
250ml milk
65g butter, room temp
7 eggs, separated
58g cake flour
23g corn flour
133g sugar
1 tsp lemon juice
¼ tsp salt
2 tbsp cocoa
3 tbsp hot water

Method
1) Prepare the cake pan but buttering the sides and bottom. Add the parchment paper to the bottom. Take a roasting tray that the pan can just fit into. Place a thin towel at the bottom so that the pan doesn't slip around. Set aside.
2) In a smaller bowl, mix the cocoa with the hot water until a smooth paste forms.
3) In a saucepan on a very low fire, melt the cubed cream cheese. Add the butter and melt. Mix these together and they will look as if they've split. Pour on the milk and blend into a smooth mixture. If necessary, sieve to get rid of lumps.
4) Beat the yolks with half the sugar until pale and fluffy. Add the cream cheese mixture and whisk until smooth.
5) Sift the flours and salt. Sift them onto the cream cheese mixture and use the whisk to fold in. Ensure no lumps.
6) In a very clean bowl, beat the egg whites until opaque and foamy. Add the sugar in three batches and beat on low so that only small bubbles are created. Periodically scrape down the bowl. Once all the sugar has been added, add the lemon juice. Turn up to high and beat until just after soft peaks but just before stiff peaks.
7) Take a third of the meringue and put it in the yolk mixture. Mix until no white remains. take the second third and this time, very gently fold in with a whisk. Pour this mixture back into the remaining third of the meringue. Very carefully fold in until no whites remain but be careful not to burst all the foam.
8) Pour half of the batter into the bowl with the cocoa paste. Use a spatula to fold until mixed.
9) Alternating the batter, pour into the centre of the pan. I've now learnt that I should pour at least ¼ of the batter at a time otherwise it will take forever! As you get closer to the centre, start pouring less and less each time. This was a messy process and some bloggers have poured the batter into piping bags which seem to make the job far less messy.
10) Drop the pan on the table top a few times to dispel the bigger bubbles. Use a skewer to trace around to burst the bubbles but try not to mix up the colours.
11) Bake at 160 deg C for 30 minutes, 140 deg C for 43 minutes, and then switch off. Leave it in the oven for 25 minutes with the door closed.

Rising nicely at 160 deg C

Continuing to rise at 140 deg C (even though some blogger said you needed the higher temps to rise!)

 12) Remove from the oven and take away the water bath. Place pan back in the oven with the door ajar for 25 minutes to dry out the bottom. Finally, take it out of the oven and let the pan cool on a wire rack until completely cool. Cover and rest in the fridge, preferably overnight.

Final 25 minutes in the oven with door ajar

Cooling on a wire rack. Shrunk from the sides and a few small cracks!

Next morning. No waist but the top shrank inwards slightly. 0.5cm uncooked bottom.. Cracks closed.

Recipe feedback:

- Very good temp control this time at 160 and 140 (slightly about the dial readings on my oven). To achieve 140, I turned it down to 0 and the temp took 5 minutes to drop the 20 degrees.) Didn't affect anything as the cake still continued to rise in the centre. No cracks observed then. Cracks only appeared after the last 25 minutes with door ajar.

- Very very slight uncooked at the bottom, about 0.5cm which could be due to the addition of chocolate. This uncooked bit had no eggy taste. Overall, somehow adding the chocolate gave the cake so much more taste! 

- Finally no waist but it slanted in at an angle! The cracks mostly disappeared the next day after resting in the fridge. Maybe I should remove the water bath once the oven is switched off, instead of waiting the further 25 minutes in the closed door oven?

- I think I will keep the timings and these temps.

Update: This blogger says that after baking, run a knife around the rim of the cake to dislodge it so that it doesn't form a waist. Could this tip be the solution to all my woes?
https://www.anncoojournal.com/recipe/cranberry-cheesecake/
So now I have to (i) drop the cake to prevent shrinking (ii) run a knife around the rim so that it doesn't form a waist or form a crack from sticking to the edge (iii) remove the water bath which I'm thinking also prevents the waist. But should I do these just after turning off the oven or just before the final 25 minutes with the door ajar?