Prepare the vegetables. Prepare the sweet potatoes. Wash and scrub each sweet potato and dry thoroughly. Transfer to the baking sheet and, using a knife or a fork, stab each sweet potato at least 6-7 times (this will vary depending on the size of your potato). Peel, seed, and chop your butternut squash into large cubes.
Preheat and boil. Preheat oven to 425 degrees F and line a large baking sheet with parchment paper. Transfer sweet potatoes to the parchment-lined baking sheet. Meanwhile, add a steamer basket to a medium pot filled with approximately 1-2 inches of water. Transfer the chopped butternut squash to the steamer basket, cover, and bring to a boil.
Bake and Steam. Transfer potatoes to the oven and bake for approximately 30 minutes. Rotate the pan 180 degrees to ensure even cooking and bake for an additional 15-30 minutes, or until you see some of the natural sugars have started to caramelize and a fork or knife is easily inserted into the thickest part of the potato. Meanwhile, steam the butternut squash for approximately 15 minutes, or until fork tender.
Puree. Once the butternut squash has finished steaming, transfer to a large blender. Remove the sweet potato from the oven and allow to rest for 5 minutes before handling. Slice the potatoes in half and carefully peel the skin away from the soft inside. Transfer the inside of sweet potato to the blender with the cooked butternut squash.* Add the milk and puree until smooth and creamy.
Season and serve. Transfer the creamy mashed sweet potatoes to a large mixing bowl. Melt in the butter and stir in the maple syrup. Season with salt, to taste, and garnish with fresh thyme, if desired. Enjoy!