In a large pot or Dutch oven, melt the vegan butter over medium heat. Add the leeks and onion to the pot, season with salt, and sauté until softened and translucent.
Stir the potatoes into the pot, along with the thyme, marjoram, garlic powder, pepper, and bay leaves. Stir to coat the veggies with the seasonings.
Pour the vegetable stock into the pot and give everything a stir. Turn the heat up to medium-high and bring the mixture to a boil. Reduce the heat to low and simmer for 20-25 minutes, or until the potatoes are fork-tender. Turn off the heat and discard the bay leaves.
Transfer the contents of the pot to a blender and blend until smooth. Add the vegan sour cream to the blender and blend until just incorporated.
Taste and season with additional salt and pepper if desired.
If your soup is thicker than you’d like, feel free to blend in a bit of extra vegetable stock.
Allow the soup to cool to room temperature before sealing it in mason jars (or another airtight container) and chilling in the fridge for 1-2 hours or until cold.
Serve chilled in bowls garnished with chopped chives.
Notes
To store: Leftover soup should be transferred to an airtight container and refrigerated immediately. It can be kept in the refrigerator for up to 3-4 days. Stir well before serving, as it can separate a bit during storage.