updated 12/07/2012 AT 11:30 AM ET
•originally published 12/07/2012 AT 9:35 AM ET
Their romance is heating up!
Joined by Dianna Agron, Emma Stone and about a dozen guests, Swift, 22, dressed demurely in black, and Styles, 18, watched as a cake was served and helped to sing along to “Happy Birthday” at the hotel’s Crosby Bar.
“Taylor and Harry [were] being very smoochie,” a source at the party told PEOPLE. “They definitely [looked] like a couple.”