Preheat your oven to 400°F (200°C). Pierce sweet potatoes several times with a fork and roast on a foil-lined baking sheet for 45 to 55 minutes until completely tender and caramelized. Let cool for 10 minutes, then easily peel away skins and transfer flesh to a large mixing bowl.

Reduce oven temperature to 350°F (175°C). Grease a 9x13-inch or 2.5-quart oval ceramic baking dish with 1 teaspoon of butter.

Using a hand mixer or potato masher, whip the warm sweet potato flesh until silky smooth. Add melted butter, maple syrup, beaten eggs, whole milk, vanilla extract, cinnamon, nutmeg, and sea salt. Whip on medium speed for 90 seconds until fluffy, aerated, and completely lump-free.

Spread the spiced sweet potato mixture evenly into the prepared baking dish using an offset spatula, smoothing the top surface.

Prepare the crunchy streusel: in a medium bowl, combine dark brown sugar, flour, cinnamon, and flaky sea salt. Add the cold cubed butter and cut in with a pastry cutter or clean fingertips until coarse crumbly clumps form. Gently fold in the chopped pecans and whole pecan halves.

Scatter the pecan streusel topping evenly over the sweet potato base in a generous layer. Bake at 350°F (175°C) for 32 to 35 minutes until the streusel is deeply golden, toasted, and bubbling gently around the edges. Rest for 10 minutes before serving warm.