I provide this data, which is freely available in chemical safety data books such as the Sigma-Aldrich Safety book, for information only and will take no responsibility for anything you may choose to do with it.
So there. (Thanks so much for the disclaimer, Em, dearie)
You would be best to use sodium cyanide or potassium cyanide for making poisoned scones but you must be certain to use baking powder in the recipe as well, (otherwise you may risk gasing yourself to death).
LDLo means lowest reported lethal dose.
appearance: white granular powder
melting point: 634.5 deg. C.
LDLo (for a human given an oral dose) 2857 micrograms per kg of body weight.
appearance: white powder with chunks
melting point: 563.7 deg. C.
LDLo (for a man given an oral dose) 6557 micrograms per kg body weight.
LDLo (for a human given an oral dose) 2857 micrograms per kg body weight
Note the difference between a man and a human. Apparently cooking will have no effect on the effectiveness of the poison.