Preheat oven to 375°F (190°C) and grease a baking dish.
Boil sweet potatoes in salted water until fork-tender, about 15 minutes.
Drain potatoes, then mash until smooth.
Stir in melted butter, heavy cream, maple syrup, vanilla, cinnamon, nutmeg, and salt.
Spread mixture evenly in the baking dish.
Bake for 25 minutes until slightly golden on top.
While baking, whisk coffee, maple syrup, balsamic vinegar, cinnamon, and salt in a saucepan.
Simmer for 5 minutes until slightly thickened.
Remove casserole from oven and drizzle the coffee glaze over the top.
Serve warm and prepare for unsolicited compliments.