The One With Joey's Award - S7-E18
Trivia: The original script for this featured a subplot with Ross seeing Chandler exiting a male strip club, with obvious confusion and assumptions, with Chandler later admitting he only goes there because he likes the sandwiches. Matthew Perry shut the idea down. He's barely in this episode, possibly as a result of this subplot being excised and it being simpler to expand the existing scenes rather than write a whole new Chandler-centric section.
Chosen answer: It sounds like you're describing a scene in S6 E3 "The One With Ross's Denial", with Phoebe and Ross when they're sitting at a table. Ross has a cookie and a coffee. Phoebe is telling Ross he's still in love with Rachel and Ross is denying it. While they talk, Phoebe grabs Ross' coffee at one point and Ross takes it right back. Then she does the same with his cookie and Ross again takes it back. At the very end of the conversation and scene, she grabs a magazine that Ross is also reading, and when he grabs the magazine back from her, she then grabs both the coffee and the muffin and starts eating the muffin.