In a small saucepan, combine sugar, cocoa, espresso, and flour with a whisk. Stir in milk and butter.
Bring to a boil over medium high heat, stirring continuously, and boil 2-3 minutes or until thickened. Remove from heat and stir in vanilla.
Hot fudge thickens a bit more as it cools. Store it in the fridge and warm up how much you want, when you want it, in the microwave. It heats up really fast, so microwave in 10 second increments.
Notes
You can use non-fat milk or 2% but whole milk is better. Your choice!
Instant espresso powder can usually be found in the coffee aisle of the grocery store.
If you find the coffee flavor to be too strong, try adding less espresso powder.