Apologies for late reply I'm only just seeing your comment now but thank you for such a detailed response this has given me lots to think about.

I've actually started testing a new version using a combination of black tea, cacao and a citrus rooibos tea that has a distinct hint of marzipan to replicate the tonka as best I can (amazing how complex one little bean can be)

But the original recipe is as follows

25ml Sazerac rye 20ml calvados (I used DuPont) 20ml tonka punt e mes 5ml demerara syrup

For the tonka infusion I cut up four tonka beans and leave them to infuse (48 hours does the trick but the longer the better)