The cake was ordered by a family member of his fiancee Perrie Edwards, who has relatives in Weymouth.
Ian Gill of Brewers Quay-based Arty Party Cakes was asked to design the cake, which features a green fist bursting through the icing.
He said: "A member of Perrie's family ordered it.
"I was told that Zayn is a fan of the Marvel Comics, especially the Incredible Hulk and Wolverine of The X- Men."
The cake, featuring the words 'Happy 21st Birthday Zayn', is a Madeira sponge.
Also designed especially for the Bradford-born singer was a 'cupcake mosaic' - a batch of delicious cupcakes that when put together make up images of the Incredible Hulk and Wolverine.
Mr Gill said: "I think it's fantastic that Perrie's family chose to support a local business and order the cake from us.
"We're really keen to see a picture of Zayn enjoying the 'hulk' cake on Twitter."
Last summer, Arty Party Cakes made a 20th birthday cake for former Weymouth schoolgirl Perrie.
The Little Mix band member made the hippie-themed cake an internet sensation after tweeting a picture of herself with it to her 3.5 million followers.
As part of the One Directioner's 21st birthday celebrations, Zayn and Perrie travelled to Disneyland Paris to meet Mickey Mouse and friends.
Last night the star was understood to have celebrated his birthday with some showbiz pals at his home in London, including bandmate Liam Payne.
Zayn was in Weymouth recently with Perrie for New Year celebrations and marked the arrival of 2014 with a new tattoo from King Street shop Tattoo Magic.