I found a recipe for chocolate mug cake that works super well and takes only ingredients you're already likely to have in your pantry and cooks in just over a minute in your microwave! And if you don't already have the ingredients, they're easy to get and then you can make mug cakes every day for like a year or something.

https://www.tablefortwoblog.com/the-moistest-chocolate-mug-cake/
(Apologies in advance if there are ads/trackers on the page—I use a blocker so I don't know.)

I substituted oat milk for regular and it still worked fine, and I found that the half-scale recipe is probably the best serving size for how rich it ends up tasting. Also, there are some other notes below the recipe card that are worth reading before you start cooking to help you understand what's going on. For example why the recipe is eggless and that regular sized cake recipes take 1–2 eggs so putting a whole egg into one mug cake is comparatively overkill. (Side note, I derived from that note that if you're making 2+ mug cakes worth of batter, mixing 1egg in there can get you a slightly spongier texture in your cakes if that's something you like.)

The page also links to other mug cake recipes derived from this one like vanilla, espresso chocolate, and apple spice, which is why I decided to try a matcha cake! It turned out really well, but next time I'm gonna borrow some ingredients from the apple spice recipe (namely, apple sauce) to make it a little less bitter and maybe use coconut milk to make it a little creamier/coconuttier. I bet a sugar frosting would be nice on the matcha cake, too, but that's more work that I'll ever personally put in for a mug cake...

One final note, the recipe says to mix the batter in a bowl before transferring to the microwave-safe mug (a 14 oz mug is ideal for the single serving, an 8–10oz mug worked great for the half serving). I successfully mixed everything directly in the mug with a fork, but I made sure to get all the crevices and stuff mixed in so there were no dry ingredients left at all. So keep that in mind if you want to dirty fewer dishes 👍