Rinse out the pot and dry it. Add the butter and bacon and cook over moderately high heat, stirring occasionally, until the bacon is crisp and golden, about 6 minutes. Add the chopped onion, garlic, celery, thyme and bay leaves and cook until the vegetables are tender, about 5 minutes. Stir in the miso. Gradually add the clam broth. Add the cream and 1 cup of water to the pot and bring to a simmer. Add the potato cubes and season lightly with salt and pepper. Simmer over low heat for about 8 minutes, until the potatoes are tender.