Place the potatoes in a large stockpot and cover with water. Generously season the water with salt. Add the bay leaves and place over high heat. Cover, bring to a boil, lower the heat and simmer until the potatoes are tender enough to smash with a fork, 25 to 35 minutes. Drain the potatoes and discard the bay leaves. Return the potatoes to the pot and add the heavy cream, sour cream and 4 tablespoons of the butter. Smash with a potato masher until smooth. Add the thyme, nutmeg and 1/2 teaspoon each salt and pepper. Stir to combine.
Place the sweet potato puree in a pot over medium-high heat. Cover and heat until warmed through, about 4 minutes.
When ready to serve, mound a third of the white potatoes in the center of a serving bowl. Next, mound a third of the sweet potatoes on top of the white potatoes. Repeat this process, alternating between the vegetables, to make white and orange circles. Swirl together to create a marbled effect showing both colors.
In a small pot, melt the remaining 3 tablespoons butter. Combine the butter and parsley. Pour over the potatoes and serve immediately.