1 twst Lemon peel (expressed and inserted)
1 Blackberry (skewered, as garnish)
Stir, strain into a chilled cocktail goblet, express and insert the lemon peel, garnish, serve with a side of soda.
Willett presents other possible ratios for the ingredients, at the link.
Andrew Willett, Elemental Mixology
Bourbon, Dry vermouth, Blackberry liqueur, Bitters, Blackberry, Lemon peel