A crosslinking agent that is an ungelled reaction product of (a) at least one aminoplast resin and (b) at least one aromatic heterocycle containing at least one nitrogen atom wherein the crosslinking agent is essentially free of active hydrogen functional groups and has a glass transition temperature of at least 10.degree. C. is disclosed. Methods for preparing crosslinking agents are also disclosed. Further provided is a curable powder coating composition including a polymer having a glass transition temperature of at least 30.degree. C. containing reactive functional groups and the present crosslinking agent; a multilayer composite coating composition and substrates coated therewith are also disclosed.