The hip-hop star went public with the unlikely project while chatting on New York radio station Hot 97 last week (ends23Oct15), revealing the odd couple have been in the studio together working on a song.

Chance stopped short of going into specific details about the track, but heaped praise on Colbert, saying, "He's amazing. I can't get that deep into it, but he's amazing."

The rapper appeared on the funnyman's The Late Show with Stephen Colbert on Monday, when he featured as the night's musical guest, and Colbert jokingly took him to task over the unexpected leak.

Sitting him down for a brief interview, Colbert said, "Chance, what part of 'on the DL (down low)' do you not understand? We agreed that we were not going to talk about it!"

An apologetic Chance confessed he had been too excited to keep the news under wraps, responding, "I'm sorry, Stephen. You're just so amazing..."

Colbert admitted he had been flattered to be approached by Chance and then revealed they have been filming the collaboration for a mini documentary.

He explained, "It was your idea. When you first mentioned doing something together, I was completely surprised, blown away, I couldn't believe that it was real, but I'm super excited that we're doing it. I'm looking forward to continue working on it. I'm excited about the little documentary we're making about it, too. All the pieces (are) coming together..."

However, the pair still have no idea when they will release the top secret track, only promising fans they will hear it "soon".

It is not yet known what the project is for, or if it will feature on Chance's upcoming debut solo album.

The rapper closed the The Late Show by premiering his new single, Angels, before releasing the track online for free.