The surfaces of glasses can exhibit liquid-like mobility while their interiors remain arrested, creating an underexplored route to crystal growth far below conventional crystallization temperatures. Here we show that the free surface of Au60Cu15.5Ag7.5Si17 metallic glass undergoes a two-step crystallization entirely below the glass-transition temperature (Tg). X-ray photoelectron spectroscopy first reveals Cu enrichment in the outer nanometers. Annealing at Tg - 20 K transforms this layer into sparse, self-limited platelets only ~4.5 nm thick, identified by X-ray diffraction and aberration-corrected STEM as metastable hexagonal-close-packed Cu. Annealing at Tg - 10 K activates a deeper liquid-like surface region in which these Cu platelets catalyze layer-by-layer growth of diamond-cubic Si-Cu crystals extending hundreds of nanometers to more than a micrometer into an otherwise amorphous glass. Thus, temperature controls not only the growth rate but also the active surface depth, phase selection, and crystal morphology. This surface-mediated pathway produces single-crystal-like nanostructures without a crystalline substrate and with minimal epitaxial constraint.