The present invention provides a crosslinking agent which may have reactive benzoxazine groups prepared from (a) at least one aromatic compound, (b) at least one aminotriazine compound, and (c) at least one other compound having active hydrogen groups reactive with aminoplast compound (b). The crosslinking agent is essentially free of hydroxyl functionality and has a glass transition temperature of at least 25° C. The present invention is further directed to a method for preparing the above-described crosslinking agent, and a curable powder coating composition comprising a mixture of (A) a polymer having functional groups reactive with aminotriazine and/or benzoxazine groups having a glass transition temperature of at least 30° C.; and (B) the above crosslinking agent. Multilayer composite coating compositions are also provided including a basecoat deposited from a film-forming composition and a topcoat over the basecoat, deposited from the curable powder coating composition described above. Coated substrates are also provided.