Preheat oven to 400 degrees. Prick sweet potatoes all over with a fork. Place on a rimmed baking sheet. Bake until very tender when pierced with a knife, 1 hour. When cool enough to handle, halve sweet potatoes. With a spoon, scoop out flesh (discard skins); transfer to a food processor. Add butter and syrup; process until smooth. Season with salt and pepper. Serve warm.
To store, refrigerate in an airtight container, up to 2 days. Reheat gently, 10 minutes.