Self-proclaimed baker Liam – the 20-year-old has changed his official Twitter handle to "liam payne the baker" – and Sophia stopped by to take a cake decorating class at the famous store, owned by Cake Boss TV star Buddy Valastro.
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Liam Payne and Sophia Smith took part in a cake decorating class at Carlo's Bakery
Bakery employee Adam told E! News that the couple's visit was "totally unknown".
Liam arrived with his pretty girlfriend by his side and proceeded to decorate a blue fondant cake with white icing, while Sophia worked on a pink and purple tie dye cake.
"They were just there to have fun. Totally spur-of-the-moment," Adam added. "They didn't get to meet Buddy, Buddy is overseas, but they met his sisters."
Liam Payne with sisters Maddalena and Mary
Such was the level of excitement in the bakery at Liam's arrival, that the only people allowed inside the couple's private class were Buddy's sisters Mary and Maddalena, Mary's two children and an instructor.
"With Liam there is was pretty crowded, it added another level of hysteria. We had a lot of fun. The bakery loved having him."
But at one point, Adam revealed, Liam "put on a Carlo's Bakery shirt and he came behind the counter and helped out some customers".
The kind-hearted pop idol also "took photos with fans and signed some autographs".