Preheat the oven to 400 degrees F.
Wash the sweet potatoes and pierce each of them with a fork to create holes in the skin. Place the prepared sweet potatoes on a baking tray. Bake the sweet potatoes for about 30 minutes, until they are fully cooked.
Once they're done, cut the sweet potatoes in half lengthwise and carefully scoop out the flesh, being cautious not to damage the skins.
Transfer the scooped-out sweet potato flesh to a large bowl and mash it using a fork.
To the mashed sweet potatoes, add yogurt, brown sugar, maple syrup, and pumpkin pie spice. Thoroughly mix all the ingredients together.
Evenly distribute the mashed sweet potato filling back into the hollowed sweet potato skins.
In the meantime, prepare the topping by combining walnuts, brown sugar, and butter in a bowl.
Sprinkle the prepared topping mixture over the filled sweet potatoes, and then bake them for an additional 15 minutes at 400 degrees F.
Once baked, serve the stuffed sweet potatoes while they're still warm, with maple syrup on the side.