Huitlacoche (or “cuitlacoche”) is a type of fungus (ustilago maydis) that has been consumed in Mexico since before the arrival of the Spaniards. 
Huitlacoche does not need to be cooked in order to be consumed, so it is generally only sautéed lightly before being added to whatever dishes it is being used in.
The number one way in which Huitlacoche is prepared in Mexico is in the famous Huitlacoche Quesadillas.