Jerry Seinfeld used to have a comedy bit about the invention of the Pop-Tart, but when his friend Spike Feresten—who wrote the famous “Soup Nazi” episode of “Seinfeld”—suggested it as a topic for a movie, even Seinfeld said “There’s no movie here.” But they workshopped the story, turning the invention of the Pop-Tart into a nutty postwar epic. Seinfeld has written films before, including “Bee Movie,” but this time he’s making his début as a director with “Unfrosted.” (The production did not, he says, have permission from Kellogg’s.) The comic talks with David Remnick about making a life in comedy, and why he continued to work so hard on his craft after retiring his massively successful sitcom. “This is a writer’s game. If you can write, you succeed. If you can't, you will not make it. . . . Any comedian can be funny onstage, but the bullets are the writing.”  And he offers thoughts on old age, as he turns seventy. “God is like, ‘I'm with you up to about thirty-eight,’ ” Seinfeld posits. After that, God says, “ ‘if you want to stay, you can stay. But I’m moving on.’ ”