Bring a large saucepan of salted water to a boil. Add the spinach and blanch until bright green, 30 seconds. Transfer the spinach to a bowl of ice water to cool. Drain, squeezing out the excess water.
In a medium saucepan, bring the heavy cream to a simmer. Pour the cream into a food processor and add one-third of the spinach. Process to a smooth puree. Scrape the puree into the saucepan. Add the remaining spinach and bring to a simmer, stirring. Stir in the butter. Season with salt and pepper and serve.