I'm getting too fucking old, the fucking etymology on this is hurting my brain. I guess I grasp the concept, but wtf does a cake have to do with it. Why can't you just say you find someone oddly attractive or w/e.
The "cake" is a literal cake, and the concept is printed pictures on sticks or straws that you put into the cake as you mention or explain the unconventional person. It's just a "viral" online trend, a random fun thing friends do.
'Mum, I've got something really important but slightly complicated to tell you. Give me three days to do some shopping, design some graphics and build accessories with them on, then bake a cake 👍'