As much as buying and eating a whole chocolate cake is 100% a bad idea for your health, doing it once is unlikely to have any noticable impact beyond making sure you have horrible shits for a day or so.

Doing it (or similar) habitually will obviously drastically increase your chances of heart attacks and other issues though of course.

But if you want to reward your inner 8 year old every once in a while I say do it, too many people make themselves permanently unhappy with dieting. The (much easier said than done) trick is very well known, and it's simple moderation.