The insertion of heteroatoms with different electronegativity into carbon materials can tune their chemical, electronic, and optical properties. However, in traditional solid‐state synthesis, it is challenging to control the reactivity of monomers, and therefore, the amount and position of heteroatoms in the final materials. Herein, a simple, scalable, and general molten‐state route to synthesize boron–nitrogen–carbon–oxygen (BNCO) materials with tunable boron–nitrogen–carbon composition, as well as electronic and optical properties, is reported. The new synthetic approach consists of polycyclic aromatic hydrocarbons (PAHs) and ammonia–borane as reactants that form a clear liquid‐state stage spanning a wide temperature range, before the solid‐state reaction. The molten‐state stage enhances the control over the synthetic intermediates and final materials, owing to improved monomer miscibility and reactivity. The BNCO composition and optical properties are tuned by the PAH selection and final reaction temperature. The advantages of this method are demonstrated herein through the tunable optical properties, excellent stability to oxidization, facile deposition on substrates, and good activity as an anode material in lithium‐ion batteries.