Sea-salt ice cream is an ice cream type found in Twilight Town and Hollow Bastion, created by Scrooge McDuck. The ice cream's flavor mixes both a salty taste with a sweet taste as well. Ansem the Wise was very fond of this ice cream, and often ate it while conversing with King Mickey, and later used it as the password for his computer in the Old Mansion.
When Sora arrives in Hollow Bastion, he sees Scrooge McDuck, who is trying to recreate the flavor for sea-salt ice cream. Through subsequent visits as the storyline advances, Scrooge comes closer to reinventing the flavor.
In one of Roxas' flashbacks, Axel offers him a stick of sea-salt ice cream before their parting. A droplet can be seen falling to the ground, though it is unknown whether the droplet is Axel's tear or just melted ice cream.
In the KH2 manga, Roxas falls off the Clock Tower while trying to catch his ice cream that was sliding off of the stick.
An interview with Nomura (http://www.kh2.co.uk/?page=NI/KH2U) shows that Nomura first tasted Sea-salt ice cream at a trip to Tokyo DisneySea, a smaller resort of Disney's parks in Japan. He liked the ice cream very much and decided to work with Disney to have it put into Kingdom Hearts II. He also said that the idea to put the dessert into the game came to him when he was thinking about what the kids in Twilight Town do and eat over summer vacation.