Learnt looks like you're 2 years old.
"Learned" makes you sound like a complete thicky without the ability to enunciate. "Learnt" is the correct spelling.
"Learned" is a word, but it is not the past-tense of "Learn". It's an adjective. "He was a learned man". A man of education.
"If you say "I learned something new today" then you're probably not a learned person".