I would try reducing the baking powder by about 1/4 tsp per 9x2” layer to strengthen the cake’s structure.  Don’t adjust the baking soda, though!

Hey Rose and Woody! So i thought i would report back with the result of my experiments! Firstly i have Bleached all purpose flour by Gold medal and according to the info printed it has about 10% protein. All this time i had been using Cake flour which i had automatically assumed was Bleached and only now discovered was unbleached!! on the failure of my white chocolate whisper cake. Julie opened my eyes to this ! Anyhow i searched around and short of orderin from the US and paying gastronomical amounts for shipping there is no bleached cake flour around these parts. So i decided to experiment with KATE flour. But by starting off with my cake flour instead ! So my cake flour has 8.4 percent protein which i still thought could be reduced?
Firstly i microwaved the unbleached cakeflour as instructed , however it took me wayyyyyyyyyy more than 10 minutes , im guessing i stood there for like atleast 25 minutes to get the flout to be at the 130 degree mark- opening and stirring every 10 seconds…I was so close to giving up considerng i was doing this at 2 in the night and i have a 10 month old at my feet.. lol ! But i just needed to know if there would be a difference!! 
I then proceeded to rehydrate the flour and then substituted cornflour for about 15 gms per 100gms of the kateflour.
I did not want to go the white chocolate whisper route again cause incase i failed did not want to waste my precious white chocolate stash!!! so decided to try the white velvet.
MY GOODNESS!!!!!!!!!!! i felt like a fake the moment that cake came out of the oven!! Not only did it not feel different , look different and smell different it tasted different and just i have not the words!! I felt like a fake for giving people cakes that i said were roses recipe and havign them rave about it when the texture was so not what rose intended to be! If they had had THIS texture i dont know what they would have done!! So magnificent!!  I was so thrilled i finally figued out what the problem was that i was calling people overseas to tell them abt my eureka moment. =/ do i sound crazy? Anyway umm , the ony downside was that there was a dome not a very large one that cracked but a dome nonetheless that needed trimming but even after the trimming i had enough heigh to slice into 2 . However i now cannot shake off the feelign that that hump should nonetheless not have been there… According to rose the hump is due to too much protein. But since i had 8.4 to begin with and i then added some cornstarch in place , shouldnt it have been low enough? Or should i b playign around with the baking powder levels next? T
ONE WORD - THANK YOU

Wow, that’s quite a story!   Sounds like you enjoyed the process of discovery.

It doesn’t sound like your dome was from too much protein, there are a few other things to check:  did you use cake strips?  Was your oven running hot?

You can try reducing the oven temp, using cake strips, and if that still doesn’t produce a cake that cools flat (it should be a little domed coming out of the oven but then flatten as it cools), then you can try increasing the baking powder just a bit.