Heat a small saucepan over medium heat. Add 1 tablespoon of the butter and swirl to coat the bottom of the pan. Add the garlic and ginger and saute, stirring occaisionally, until the garlic is brown, 3 to 4 minutes. Add the cream and reduce by 1/3 over low heat, about 10 minutes. Keep warm. Meanwhile, combine the sweet potatoes in a large saucepan with enough water to cover them completely. Bring to a boil over high heat and cook until they can be pierced easily with the tip of a knife, 20 to 30 minutes. Drain and transfer to a food processor. Add the cream and process until well blended. Add the remaining butter, season with salt and pepper, and puree until smooth. Transfer the potatoes to a bowl, correct the seasonings, and serve.