EDIT: Now with image!
The mochi cake I used is from a mix (I like the one that Trader Joe's sells), but it you wanted to make your own I don't think it's too complicated. I like a thinner cake, so I usually only use half a box of the mix. Doing that also means it cooks very quickly!
What takes it to the next level, in my opinion, is to get some freeze-dried strawberries or other freeze-dried fruit (Trader Joe's also sells this, and freeze dried fruits of all kinds are common in most supermarkets, usually in a snack aisle), grind them up, and dust them over top like you would with powdered sugar, but in a thicker layer. I like to grind mine with a mortar and pestle, but a (blade-style) coffer grinder or anything like that works too.
It's honestly the simplest desert recipe I know, and one of my favorites!
Yeah! The availability of freeze-dried fruits makes it really easy (although they do suck up humidity from the air over time after you grind them, so you'll want to eat the cake within a couple days ๐)