There is no such thing as 'A PANINI' because the word 'panini' is plural, 2 or more rolls in the bella lingua italiana (beautiful Italian language). A single sandwich of that ilk is a 'PANINO.' Actually, in order to make it a sandwich, it should be called a 'PANINO IMBOTTITO' which is a filled roll.
Why do I rant? Because it has become commonplace in the US to screw up a beautiful language like Italian.
Also, there is no need to end the following Italian words with an 's' because they are already plural...spaghetti, panini, ravioli, cannoli, etc. I've heard these words with an 's' attached in South Philly. Oy vay!