In a large pot, bring the chicken broth and salt to a boil. Add the cauliflower; bring back to a boil. Cover, reduce the heat to low and steam for 20 minutes, or until cauliflower is very tender. Use a slotted spoon to transfer the cauliflower to a food processor. Add 3 tablespoons of chicken broth from the pot, along with the butter. Process until smooth. Taste and adjust seasoning with salt and freshly ground pepper. Add thyme and process until just combined.