Preheat the oven to 400°F. Peel and cut sweet potatoes into 1-inch cubes, spread on a baking sheet, and roast for 25 to 30 minutes until tender.

Transfer roasted sweet potatoes to a large mixing bowl and mash with an electric mixer on medium speed for 2 to 3 minutes until mostly smooth.

Add 4 tablespoons of butter, heavy cream, and whole milk to the hot sweet potatoes and mix on medium speed for 1 to 2 minutes until creamy and smooth.

Stir in vanilla extract, cinnamon, nutmeg, salt, and black pepper with a wooden spoon until fully incorporated.

In a medium mixing bowl, combine pecan pieces, brown sugar, and the remaining 2 tablespoons of butter until the mixture resembles coarse breadcrumbs, then drizzle in maple syrup and toss to moisten.

Butter or spray a 9x13 inch baking dish, pour in the sweet potato mixture, and spread into an even layer.

Evenly sprinkle the pecan topping over the sweet potato base and press gently to adhere.

Reduce the oven temperature to 350°F and bake the casserole for 20 to 25 minutes until the topping is golden brown and the edges are set.

Remove from the oven and let cool for 5 minutes before serving.