Preheat oven to 450°F. Place sweet potato, Yukon Gold potato cubes, and shallots in a large bowl. Drizzle with extra virgin olive oil. Sprinkle with sea salt, fresh cracked black pepper, thyme, and ground cumin. Toss until ingredients are well coated with olive oil and seasonings.
Spread the potatoes and shallots out on a rimmed cookie sheet brushed with sweet cream butter or EVOO. Roast for 25 minutes or until cooked through and nicely caramelized and browned around the edges.
As the potatoes and shallots are roasting, heat chicken stock in a saucepan on the stove-top until steamy. You'll see wisps of steam coming off.
Remove the cooked potatoes and shallots from the oven, and with a slotted spoon, add a quarter of the mixture into the blender, then add 1/3 the hot chicken or vegetable stock. Purée until smooth. NOTE, when blending hot liquids, work in batches and remember only to fill the blender container a third full, and use short pulses. Repeat until all ingredients are used. Repeat the process until all the ingredients are blended.
Adjust taste if needed by adding sea salt and fresh cracked black pepper. Serve in bowls with a bit of sour cream and garnish with a sprig of fresh thyme (both optional).