Preheat the oven to 400°F and line a baking sheet with foil. Halve the butternut squash lengthwise and scoop out the seeds and any loose fiber. Lightly coat the cut faces with the butter. Place cut-side-down on the baking sheet and bake until a knife goes through the skin easily, 1 hour to 1 hour 30 minutes. Let the squash cool completely, then scoop out the flesh and set aside. Discard the skin.
In a large, deep saucepan, heat the olive oil over medium heat. Add the onion and cook until translucent, 3 to 4 minutes. Add the garlic, thyme, and parsley and cook for 1 minute more.
Add the flour and cook, stirring, for 1 minute. Slowly add 2 cups of the vegetable broth, stirring constantly. Add the baked butternut squash and stir well. Stir in the remaining stock and simmer for 10 minutes. Remove from the heat.
Use a hand blender to puree the soup in the pan until it's smooth. (Alternatively, in batches, puree the soup in a blender or food processor and then return to the pan.)
Return the soup to medium heat and bring to a boil. Add the kale, salt, pepper, and sugar. Simmer until the kale is tender, 5 to 10 minutes. Serve hot.
