Place a Dutch oven over low heat. Add the butter and melt. Add the celery, onion and garlic and sweat until tender and translucent, 8 to 10 minutes.
Add the stock, chestnuts, sherry, heavy cream, bay leaf and thyme, bring to a gentle simmer and cook until the chestnuts are very tender, 10 to 15 minutes.
In the meantime, whisk together the creme fraiche and pumpkin pie spice in a small bowl until combined. Set aside for garnish.
Remove the thyme sprig and bay leaf from the soup. Carefully transfer the soup to a blender and puree on high speed until very smooth (raise the speed of the blender gradually and leave the lid ajar to allow the steam to escape). Sprinkle with salt and pepper to taste, thinning with more heavy cream as desired and a splash of the sherry. Serve warm, garnished with the spiced creme fraiche.
When blending hot liquid, first let it cool for five minutes or so, then transfer it to a blender, filling only halfway. Put the lid on, leaving one corner open. Cover the lid with a kitchen towel to catch splatters, and pulse until smooth.