When I was a college student, I found my sanity in wax paper cups of ice cream with hot fudge sauce from Herrell's in Harvard Square. It was the kind of treat that was small enough not to seem overly unhealthy — my grandma eats ice cream with hot fudge sauce every day, and she’s 93 — but large enough to assuage any stress that exams, professors, or snow might contribute to my life. I ate ice cream all winter, all fall, and all spring.
Combine the cream and the butter in a small, heavy saucepan over medium-low heat. Cook, stirring occasionally, until the butter is melted and the mixture has just come to a boil. Stir in the sugars and the pinch of salt and cook for another minute, letting the mixture barely simmer. You want the sugars to be dissolved, and you can test this by spooning a little bit onto a plate and then rubbing it in between your fingers to make sure it’s smooth, not gritty.
Add the cocoa powder slowly, whisking as you go to stir out any lumps. The sauce should be uniformly smooth, thick, and dark.
Take the sauce off the heat. Add the chocolate, vanilla, and espresso powder. Let sit for about a minute, then whisk again until smooth.
Serve immediately, over ice cream or your dessert of choice.