BorK
>exception for 10
False.
I'm thinking that as long as there are two numbers/digits, they have to be added together. Stop when there is only one digit left.
So 11 would go to 2. Answer to BorK is B.
DBEIAC ECFBBH
Now thinking over the next step.
This puzzle itself has to have a plaintext solution to light the way for what should be done to the boomtext.