How to Make It
In a large saucepan, cover the carrots with the sparkling water and bring to a boil over moderately high heat. Stir in 3 tablespoons of the butter and 2 teaspoons of salt. Reduce the heat to moderate and simmer until the carrots are tender when pierced with a knife and the cooking liquid is reduced to 1/4 cup, about 25 minutes.
Transfer the carrots and their cooking liquid to a food processor and let cool for 10 minutes. Add the remaining 1 tablespoon of butter and process until smooth. Add the chopped cilantro and pulse until just incorporated. Season the carrot puree with salt and pepper, transfer to a bowl and serve.