There is no Direct Bus between Dhaka to Bhutan. If you want to travel from Dhaka to Bhutan by road, you have to go through India. Best route to reach Thimphu or Paro from Dhaka is to take an Indian transit visa. Then travel Dhaka to Burimari by bus. After immigration formalities, Cross border from Burimari to Changrabandha, India. Then hire a car to travel from Changrabandha to Jaigaon, Indian border town with Bhutan. After having immigration in Jaigaon border, walk from Jaigaon to Phuentsholing. In Phuentsholing, you have to take on arrival visa for Bhutan. But, to get visa, you have to fill up a form and submit 1 Photo, hotel booking and passport photocopy. Bhutan immigration office will immediately issue tourist Visa. From Phuentsholing you have to ride on Bus or hire a car to reach Thimphu or Paro city.