business as usual, in other words
[Sebastian notices the chocolate decrease around them, but doesn't care about it right in the moment. Ciel is what matters. Sebastian doesn't attempt to stop his leaning away, but neither does he remove his arms from the underside of Ciel's, nor moves away from his kneeling position before Ciel. It makes for an intimate - and ironic - position for little heart-to-hearts like this.
Ah, the young master doesn't trust Sebastian that much. Was Sebastian only trying to mislead him? Well, whatever he had planned - or considered - his answer to be, Ciel's response cements it. If the young master wants Sebastian to be untrustworthy, to be unpredictable, then that's what Sebastian will be. His smile widens fractionally.]
I could swear on my honour, or on a Bible, or some such. I believe that is what someone not a demon would do?
[Get it? Sebastian lacks both the human version of honour and a belief in any god, and humans swearing by either of those things means absolutely nothing in terms of guarantee.
"You would have no guarantee" is what he's saying. That's the answer Ciel wants, isn't it?]