Unlike most food, the cake cannot be eaten as an item in the hotbar. Before being eaten, it must first be placed on top of a solid block. Placing the cake on a slab also works, as the slab acts like a solid block. The cake instead floats half a block on top of the slab.
Each cake has seven "slices"; each use consumes one slice progressing inward from the west. A single slice restores 2 hunger and 0.4 hunger saturation. Eating all seven slices of a cake restores 14 hunger and 2.8 hunger saturation.
Read more about cake on https://minecraft.fandom.com/wiki/Cake