Decadence of the word decadence
Change in meaning, or semantic drift, can occur over time through many mechanisms: changes in reality (like technology), metaphorical usage slipping into dead metaphor, idiosyncratic error that gets propagated, just random emphasis in different locations, among many .
The use of 'decadent' has a reasonable travel path from its nominal meaning of 'in a state of decline' to particularly luxurious or exorbitant but available to non-upper class to self-indulgent. This particular path seems metaphorical to me given that one part of decay or decline might be favoring luxurious practices beyond ones means.
As to timing, Etymonline gives around 1970 as the start:
Beckoning sense of "desirable and satisfying to self-indulgence" begins c. 1970 in commercial publications in reference to desserts.
It does not specify this usage as especially American. As an AmE speaker, I don't find the usage for rich food to be strange, but it does always sound a little excessive to me.
Etymonline says that this has been going on for forty years, mentioning that the word is "usually used in a bad sense," but:
Beckoning sense of "desirable and satisfying to self-indulgence" begins c.1970 in commercial publications in reference to desserts.
Four decades is long enough for some dictionaries to catch up; one of Macmillan's definition reads:
allowing yourself, or providing, so much pleasure that it almost seems morally wrong : a deliciously decadent dessert
The Cambridge online dictionary specifically denotes this usage as "humorous":
decadent (adj.) A decadent person or group has low moral standards
a decadent society
the decadent court surrounding the kingHUMOROUSChampagne and chocolates for breakfast - how decadent!
The closest Collins comes to portraying this word in a positive light is including self-indulgent in its definition; however, the surrounding language doesn't make it seem like the word can be used in such a neutral or "humorous" way:
decadent (adj.) characterized by decay or decline, as in being self-indulgent or morally corrupt
with these synonyms:
degenerate, abandoned, corrupt, degraded, immoral, depraved, debased, debauched, dissolute, self-indulgent
Getting back to your question, I'd surmise that decadent started being used in this way – that is, as a positive descriptor of food – around the same time the term self-indulgent didn't necessarily imply moral lapses.
Interestingly enough, the McDonald's ad campaign You Deserve a Break Today began in 1971; around the same time Etymonline says that advertisers started proclaiming that it was okay to be decadent. Evidently, we deserve a little self-indulgence now and then, and partaking does not make us morally corrupt – at least, that is what the advertisers would have us believe. The OED, though, is not so quick to agree:
decadent (adj.) That is in a state of decay or decline; falling off or deteriorating from a prior condition of excellence, vitality, prosperity, etc.
with no added mention or example citations of the word being used to "humorously" describe chocolate. I'd point out that the OED meaning seems to accurately describe what might happen to a waistline after guiltlessly partaking in too many decadent desserts.
In the context you describe the value of the word decadent lies not in its meaning but in its associations.
Many people who cannot define the word would be able to tell you that it has to do with ancient Rome -- reclining on couches, being feed delicious morsels by nubile slaves, feasting on exotic foods, all without exerting any apparent effort in preparation.
That sounds like every ad-man's vision of a frozen meal to me!