India is the answer, in the very first centuries temples built over there were actually 90% Indian style. Angkor was built in 9-13th century, with still a mix of Indian and Khmer style, the royal, political, social castes were Indian styles.
Japan and Korea on the other hand were influenced by China, also Vietnam which located in Southeast Asia, southern part of China as well.
There's a theory of Cambodia (Kampuchea = local name) was actually a lost tribe descended from Persia as well called Kamboja. This was applied because of the sound of the name and a folklore about how the country was created.