Just the way I learned it, but worth a try: mix bourbon/rye with syrup, pour over a large glass/jar containing a lot of mint. Muddle mint in the liquid, cap & refrigerate overnight. The liquor tames the mint, the mint tames the liquor, the syrup just makes it taste like just refreshing enough. Strain liquid onto ice & stir. I never can get the crushed ice crushed enough . Serve in a sterling julep cup, sip while watching the Derby on the tube.
I do my own "Vesper in Absinthia" by pouring the once-made Vesper into a chilled, absinthe-rinsed glass. I like the body that the vodka somehow adds in the Vesper, I just like the absinthe favor, and I am ever the punster, here on Vesper (& Ian Fleming) no longer being with us, plus the "in absentia" phrase frequently being applied both to trials and awards.
Giffard Caramel Toffee Liqueur?! This is something I've never heard of, much less have in my larder. I understand why you do this kind of thing, and you do it continually, but your recipes would be tried more if you offered substitutes or how-to-make-ersatz-versions of uncommon ingredients. That way, if I liked the jury-rigged version, I'd be much more likely to go for the real thing - and Giffards would sell more. As it is, this recipe prompted me to make an Old Fashioned, period.
I approximated the recipe (no myrtle here, so used blueberry-infused vodka - 3 days infustion), but the lime is much, much more powerful than the blueberry, so while tasty, it came out more limey than berry-y.