A little confused. As I know, Boten is connecting the border at Yunnan Province(https://www.travelchinaguide.com/cityguides/yunnan/) of China. From Nanning, you could go to Vietnam, not Laos.

So, from Boten, you could take a bus to Kunming City(https://www.travelchinaguide.com/cityguides/kunming.htm) of China and then fly to Hong Kong directly. Of course, you could go to Guangzhou firstly and then transfer to Hong Kong by bus or train in 3 hours.