It is a large grey cake in a tombstone shape with black icing. Written on the cake is the inscription:
Sir Nicholas de Mimsy-Porpington
died 31st October, 1492
It is not meant to be edible by humans.
The Deathday Party cake is not meant to be eaten but walked through by the ghost guests, who believe they can smell or taste rotting (i.e. almost dead) food.
It is from the date of 1492 on this cake, that the timeline of the Harry Potter novels can be reckoned. Since this is the 500th anniversary of Nearly Headless Nick's death, the year would be 1992. Therefore, Harry's first year at Hogwarts would be in 1991.
From the Web
Writing by J K Rowling on WizardingWorld.com (Pottermore): Hogwarts Ghosts
Harry Potter Wiki: Tombstone cake
Feast of Starlight cake recipe: Sir Nick’s Deathday Party: Tombstone Cake