Whisk together milk, cocoa, cornstarch, and coffee in a small heavy saucepan until smooth. Whisk in agave. Whisking constantly, bring to a simmer over low heat; scrape bottom of pan frequently with whisk to prevent sauce from sticking and scorching. Continue to simmer, whisking constantly, for 1 minute. Remove from heat and stir in chocolate chips and salt until chips are melted and smooth.