No I didn’t dip-dye my cake, but it’s the same premise, gradually fading coloured icing, Pinterest is flooded with them. I think it’s more dramatic with a colour but I was asked for a chocolate cake so I just added more cocoa. Had I have been doing a taller cake I could have made a layer cake of progressively more chocolatey sponge layers too, a challenge for another day. I just used a basic sponge recipe as I had done for my Bundt Cake and cupcakes, a really versatile recipe to easily master.
I’d advise separating your starting colour, the weakest one into 3 bowls and them adjusting the cocoa accordingly. I was tempted to slap the icing on and just keep adding cocoa to my one vat as I went along but I think determining all the shades first means you can get it all on and not have to scrape it off and start over if it’s not to your liking. The technique is simple, pipe even sized blobs in a row and flatten with a palette knife or equivalent, then repeat so they look a lot more uniform than mine.
I wanted a perfectly tempered chocolate disc, I didn’t do any prep despite my desire so spread some (unknown temperature) melted chocolate onto a tightly clinged deep plate. Using the rim as a guide you can get an un-messy near accurate circle and the elevated cling makes for a flat surface once set and turned over.
Unfortunately, I stored my cake in a not-cooled-enough oven overnight and my beautiful chocolate disc dried out, though did nicely hug the finished cake well. I can assure you it looked as described before until my ill thought out storage method. Still a minor triumph in celebration cakes.