Learned is the standard past-tense verb for both American and British English. Although the variant, learnt, appears more frequently in British English, it is still so uncommon that it can safely be considered an affectation best avoided. Don’t be distracted by Bryan A. Garner’s vague description that, “in BrE, it’s [the verb] often learnt,” which seems to imply that British English writers prefer that spelling—they do not.
Pronounce learned with one syllable when it’s a verb and two syllables when it’s an adjective.