Self-organized Co platelets with a singular height and of equilateral triangular shape are fabricated on a Si(111)-7Ã-7 surface pre-decorated with an periodic array of Al magic nanoclusters, as observed using the scanning tunneling microscopy. The selection of such a singular height is attributed to the suppression of chemical reaction between Co and Si by the Al nanoclusters, and the corresponding improved confinement in the motion of the conduction electrons within the Co platelets. Such quantum platelets exhibit intriguing magnetic properties in the hysteresis loop, which is shown to reflect a synergism between magnetic dipole interactions and magneto-crystalline anisotropy of the platelets. The present study demonstrates a promising pathway to directly integrate magnetic nanostructures with Si-based electronic devices. @FootnoteText@ In collaboration with Ming-Hu Pan, Hong Liu, Jun-Zhong Wang, Jin-Feng Jia, Xiang-Rong Wang, J. T. Markert, C. K. Shih, Zi-Qiang Qiu, and Zhenyu Zhang.