Tried this excellent bourbon infusion at Alston's under central station in Glasgow.
2oz Tonka Bean Infused Makers Mark
1/4oz Maple syrup
2-3 dashes angostura
Orange peel garnish
Combine angostura and maple syrup in mixing glass before adding Tonka bean infused bourbon. Fill 3/4 with ice and stir until diluted and chilled. Strain into an old fashioned glass over a large rock of ice and run orange peel around the rim before adding it to the drink.
The staff on duty could not tell me the proportion of Tonka bean used in the infusion and I've yet to try it myself but I recommend experimentation. My Tonka beans are already in the mail.