Burned and burnt both work as the past tense and past participle of burn. Both are used throughout the English-speaking world, but usage conventions vary. American and Canadian writers use burned more often, and they use burnt mainly in adjectival phrases such as burnt out and burnt orange. Outside North America, the two forms are used interchangeably, and neither is significantly more common than the other.
Burned is the older form. Burnt came about during a period in the 16th through 18th centuries in which there was a trend toward replacing -ed endings with -t in words where -ed was no longer pronounced as a separate syllable. Later, British writers continued to favor the newer -t forms for a handful of verbs, while North Americans went back to the more traditional -ed forms.
The below ngram graphs the use of burned and burnt in American-English books published from 1800 to 2000. It shows that burned has been preferred in the U.S. for well over a century.
And this ngram graphs the use of the words in British English during the same period:
Of course, these graphs don’t indicate how the words are used, but they do show changing usage patterns. Burned appears to have recently gained the edge in British writing.
Outside North America, writers use burned and burnt interchangeably, as in these examples from British, Australian, and Irish sources:
The book was ceremonially burnt by O’Brien’s local priest with her mother’s full agreement. [The Telegraph]
They were found burned at Free Derry corner in the Bogside. [BBC News]
England’s poor use of the controversial decision review system burnt them badly. [Sydney Morning Herald]
Baddeley burned the rim on a short putt that would have sealed the win. [Irish Times]
Writers in American English usually use burned-–for example:
Deputies said a woman burned a man with a hot bowl of soup. [Kiro TV (article now offline)]
It’s Larry, in his more familiar severely burned visage. [Wall Street Journal]
Colorado Springs firefighters have a brush fire that has burned about 50 acres on the southern tip of Fort Carson overnight about 75 percent contained. [Colorado Springs Gazette]
And in American English, burnt is almost exclusively used as an adjective, often in the phrasal adjective burnt-out and in other adjectival phrases—for example:
The kitchen cabinets are painted burnt orange in honor of the Texas Longhorns. [Wall Street Journal]
A police car stands in front of the burnt-out remains of the apartment. [CNN]