1 bun for 2,000 won (expensive!)
Their slogan: "It's never enough to have one!" (It's true)
Rotiboy has more choices than Deli Roti, has kaya bun and others. May be I will try next time. Anyway, nowadays, Korean bakery also sell mocha bun but it is not as delicious as this shop.
There is a branch near to Guui subway station (line 2), just next to Gwanjin office.
At first this shop was available just next to Rotiboy branch at Gwangjin area, it was closed up. I found it again at Jamsil station - a small shop inside the Kyobo bookshop.