It's an ongoing gag of the Buono members to put their fingers on their cheek and say "Buono desu!" or "Buono deshita~!" Actually, I'm not sure if it's a gag, but I do know they always do that whenever they introduce themselves, haha.
It's a gesture used (i believe) in Italy to show that something tastes good. And Buono! Has that whole blog basically dedicated to food, so it makes sense that they'd use such a gesture.
Comments 
