We demonstrate a novel, to the best of our knowledge, mechanism for generating half-cycle unipolar light pulses through reflection of an intense single-cycle femtosecond laser pulse from a transient plasma layer. The leading pulse edge ionizes a thin medium layer, while the trailing edge reflects from the plasma boundary, producing subcycle unipolar waveforms. Our 1D Maxwell-plasma model reveals universal scaling laws for pulse shape versus plasma density and thickness, showing how thin layers yield isolated half-cycle pulses, while thicker layers produce rectangular waveforms. This all-optical approach enables attosecond field synthesis without complex phase-matching requirements, bridging classical electrodynamics and ultrafast optics for quantum control applications.