Bake the sweet potatoes:
Preheat the oven to 400°F (200°C). Pierce the sweet potatoes several times with a fork and place them directly on the oven rack. Bake for 45–50 minutes, or until the potatoes are tender when pierced with a fork.
Prepare the filling:
Once the sweet potatoes are cool enough to handle, cut them in half lengthwise. Scoop out the flesh into a large bowl, leaving a small border around the edges so the skin remains intact. Discard the skins or save them for composting.
Mix the filling:
To the sweet potato flesh, add the softened butter, brown sugar, cinnamon, nutmeg, vanilla extract, and milk. Mash everything together until smooth and creamy. Season with salt and pepper to taste.
Prepare the streusel topping:
In a separate bowl, combine the softened butter, brown sugar, flour, cinnamon, and chopped pecans. Mix until the ingredients form a crumbly texture.
Stuff the sweet potatoes:
Spoon the sweet potato mixture back into the potato skins, dividing it evenly between the four halves.
Top with streusel:
Evenly sprinkle the pecan streusel mixture over the top of each filled sweet potato half.
Bake again:
Place the stuffed sweet potatoes on a baking sheet and return them to the oven. Bake for an additional 15–20 minutes, or until the topping is golden brown and the filling is heated through.
Serve:
Serve these twice-baked sweet potatoes as a side dish with your favorite main courses, or enjoy them as a decadent vegetarian meal.