From the Super Mario Wiki
Hot Sauce is an item that can be obtained in Paper Mario: The Thousand-Year Door after completing the Businessman's security code trouble after Chapter 6. More can be purchased from the Ratooey Businessman for ten coins apiece. The item will charge the user's attack power by one point. Unusually, it can be mixed with a Cake Mix to make a Fire Pop.
In Super Paper Mario, players can get them by buying them at the Itty Bits store in The Overthere for 20 coins. Sometimes enemies such as Cheep Cheeps and Clubbas drop them on defeat. When used, it doubles attack power for 10 seconds. It makes sense for Clubbas to drop them, as they also commonly drop Fire Bursts, which is the result of cooking a Hot Sauce.
The Hot Sauce item bears a striking resemblance to a real-life bottle of Tabasco sauce.