Spherical robots typically require at least two actuators to achieve controlled 2D planar motion. Here we present Rollbot, the first spherical robot capable of controllably maneuvering on a 2D plane with a single actuator, challenging this assumption. Rollbot rolls on the ground in a circular pattern and controls its motion by changing the trajectory's curvature by accelerating and decelerating its single motor and the attached mass according to our derived quasi-stable state dynamics and control laws. We present the theoretical analysis, design, and control of Rollbot, and demonstrate its ability to move in a controllable circular pattern and follow waypoints, validating the efficacy of the proposed theoretical framework.