Preheat your oven to 350°F and line a baking sheet with foil. Pierce whole sweet potatoes several times with a fork and roast them for 40-45 minutes until completely tender and easily pierced. Remove from oven and let cool slightly until you can handle them comfortably.
Scoop the flesh from roasted sweet potatoes into a large mixing bowl, discarding skins. Mash with 4 tablespoons butter and heavy cream until smooth and creamy, then stir in vanilla extract and salt until combined thoroughly.
Add cinnamon and nutmeg to the sweet potato mixture, folding gently to distribute the spices evenly throughout. Stir in 1/4 cup brown sugar and taste, adjusting sweetness if needed for your preference.
Spread the creamy buttery pecan sweet potato casserole recipe into a buttered 9x13 baking dish, smoothing the top with the back of a spoon. In a small bowl, combine chopped pecans, 3 tablespoons melted butter, 2 tablespoons brown sugar, salt, and maple syrup until clumpy and fragrant.
Scatter the pecan topping evenly across the sweet potato layer, pressing gently so pieces adhere to the surface. Bake for 20-25 minutes until the topping turns golden brown and the filling is heated through completely.
Remove from oven and let rest for 5 minutes before serving, which allows flavors to settle and makes portioning easier.