Soak chile in warm water in a 1-cup glass measure until softened, about 30 minutes, then drain.
Finely chop cilantro and garlic together in a food processor. Add chile, lime juice, sugar, and salt, then process until finely chopped. Add butter and blend until smooth, then spoon into a ramekin or roll into a cylinder using parchment paper.
Cooks' note: Garlic-ancho butter keeps, tightly covered and chilled for 1 week