A crosslinking agent having reactive benzoxazine groups is provided. The crosslinking agent is an ungelled reaction product of aminoplast resin, a polyhydric aromatic compound, and at least one compound having active hydrogen groups reactive with aminoplast resin which is selected from at least one of: (i) compounds having the structure (I): where X is aromatic; R1, R2, and R3 each independently represents H, (cyclo)alkyl having from 1 to 12 carbon atoms, aryl, alkaryl, aralkyl, or an active hydrogen-containing group; (ii) compounds having structure (II) or (III): where R′ and R″ each independently represents an aromatic group or an alkyl group having 1 to 12 carbon atoms; and (iii) compounds different from (i) and (ii) and having a melting point of at least 80° C. Curable powder coating composition and multilayer composite coating compositions are also provided, as well as coated substrates.