Looked into it. It’s from s6e8 “The Crash” the one where they get injections to boost creativity.

At the agency, Don pauses while watching Peggy comfort a bereaved Ted before the stimulant suddenly takes effect. That evening, Don rummages through a magazine pile and asks for Ken. He tells Ken he is confident he can create a winning Chevy ad campaign but needs to see the executives in person to present the work. Ken begins tap dancing during their manic conversation.