Preheat oven to 350°F (175°C). Lightly grease a 9x13-inch ceramic or glass baking dish with butter.

Place cubed sweet potatoes in a large pot of cold salted water. Bring to a rolling boil over medium-high heat and cook for 15-20 minutes until fork-tender. Drain thoroughly and transfer to a large mixing bowl.

Mash sweet potatoes with a potato masher or hand mixer until completely smooth and lump-free.

Add 1/3 cup melted butter, pure maple syrup, whole milk, beaten eggs, pure vanilla extract, ground cinnamon, nutmeg, and sea salt. Beat on medium speed for 1-2 minutes until light, fluffy, and thoroughly combined.

Spread the sweet potato mixture into an even layer in the prepared baking dish.

Prepare the Pecan Streusel: In a medium bowl, combine chopped pecans, dark brown sugar, all-purpose flour, and a pinch of salt. Add cold cubed butter and cut in using a pastry cutter, fork, or your fingers until crumbly and pea-sized clumps form.

Scatter the pecan streusel evenly over the top of the sweet potato base in a generous, thick layer.

Bake uncovered at 350°F for 35-40 minutes until the casserole is hot and bubbling around the edges and the pecan topping is fragrant and deeply golden-brown. Rest 10 minutes before serving warm.