Cheese

Cheese is a dairy product which first appears in Luigi's Mansion. Luigi can scan cheese with the Game Boy Horror to reveal a Gold Mouse, which he can suck up to get treasure.

Cheese later appears in Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey, where it can be eaten by Bowser to restore BP, replacing Syrup Jars from the previous game.

Luigi's Mansion
In Luigi's Mansion and the remake, Luigi can scan cheese with the Game Boy Horror's Search Function to reveal a Gold Mouse, which he can suck up to get treasure, including jewels.

Five pieces of cheese appear in the game. They are found in the following locations:
 * Study - Behind the desk
 * Fortune-Teller's Room - Behind Madame Clairvoya's chair
 * Dining Room - Under the chair next to Mr. Luggs
 * Tea Room - Behind the left table
 * Safari Room - Between the crates and the armchair

Cheese only appears in darkened rooms, and disappears when the room is lit up. However, unlike with Speedy Spirits, Luigi gets a second chance to catch the Gold Mice he missed, during the Blackout.

In Neville's Study, there is a book called Darkness is Their Cheese by C. Parmesan that talks about the effects of cheese in the dark on Golden Mice:

Ghost mice are drawn to darkness as real mice are drawn to cheese... In the dark, they shine gold with contentment. I must try to get close to one before the lights come back on.

Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey
In the Nintendo 3DS remake of Mario & Luigi: Bowser's Inside Story, Bowser uses cheese to restore BP instead of Syrup Jars. They recover fifteen Brawl Points when eaten. There are stronger variants of regular cheese: Super Cheese, Ultra Cheese, and Miracle Cheese. Also, Bowser's equivalent of the Star Candy is the Cheesy Drumstick.