Combine the dried mushrooms and 1 cup of hot (but not boiling) water in a bowl. Place something weighty on top of the mushrooms to keep them submerged.
Heat the vegan butter in a heavy pot over medium heat. When melted, add mushrooms. Saute until tender, 5-6 minutes. Add onion and garlic. Saute until onion is translucent, 2-3 minutes. Season generously with salt and pepper. Sprinkle in the flour and stir well until the ingredients are coated with flour and the flour is slightly toasted, 1-2 minutes. Add sherry (or the 1/2 cup of vegetable stock) and scrape loose any stuck bits.
Once the mushrooms have rehydrated for at least 20 minutes, combine the mushrooms, the mushroom soaking water, cashew butter, 1 cup vegetable stock, the plant milk, and the sauteed mushroom-onion-garlic mixture (see note on keeping whole mushrooms) in a high-speed blender. Blend on high until smooth, 2-3 minutes, depending on your blender.
Wipe any solid bits out of the pot you were using and transfer the blended mixture and any reserved mushroom pieces into it. Add the bay leaf and soy sauce and turn the heat to low. Cover and simmer for at least 10 minutes (see note), stirring occasionally, to infuse the bay leaf flavor.
Before serving, stir in (truffle) salt until the soup is sufficiently salty. Garnish to taste with croutons, parsley, and a drizzle of olive oil and serve immediately.