Preheat oven to 375°F / 190°C
Butter a 9×13-inch metal baking dish
Simmer sweet potatoes, brown sugar, granulated sugar, hot water, butter pieces, cinnamon, nutmeg, and salt for 10–12 minutes until potatoes are just tender
Stir in vanilla extract
Pour potatoes and syrup into baking dish and scatter butter on top. Do not stir
Whisk together flour, sugar, baking powder, salt, milk, and melted butter until smooth and pourable
Pour batter evenly over hot filling. Do NOT stir
Bake uncovered 45–55 minutes until golden and aggressively bubbling
Rest 5–10 minutes before scooping
Serve warm with vanilla ice cream or evaporated milk
