Preheat your oven to 350 degrees. Butter a 9x13 inch baking dish and set it aside.

Place the sweet potato chunks in a large pot and cover with cold water by about an inch. Bring to a boil over high heat, then reduce to a steady simmer. Cook until a fork slides in with zero resistance, about 18 to 22 minutes. Drain well and return the potatoes to the hot pot for two minutes to let any extra moisture steam off.

Add the softened butter, brown sugar, vanilla, cinnamon, nutmeg, and salt to the drained sweet potatoes. Mash or beat with a hand mixer until smooth. Pour in the room-temperature buttermilk and mix until the filling is creamy and uniform. Taste and adjust salt if needed. Spread the filling evenly into the prepared baking dish.

Make the streusel. In a medium bowl, combine the flour, brown sugar, cinnamon, and salt. Add the cold butter cubes and work them into the flour mixture with your fingertips, pressing and squeezing until the mixture looks like coarse crumbs with some pea-sized bits. Stir in the chopped pecans.

Scatter the streusel evenly over the sweet potato filling, covering it all the way to the edges.

Bake uncovered for 35 minutes, until the streusel is deep golden brown and the filling is set and pulling away slightly from the sides of the dish. Let it rest for 5 minutes before scooping and serving.