Cupffee, a start-up making coffee cups from oats and wheat, is on a mission to stimulate demand for ‘sustainable edibles’. Disposable single-use coffee cups are bad news for the environment.

Combine the sugar, cornstarch, and salt in a medium saucepan. Add the milk and egg yolks and whisk together. Place over medium heat and cook, stirring continuously with a rubber spatula, as the ...