Citrus sourness mellowed by a touch of sugar and invigorated by a blast of toasty bourbon whiskey.
Many assert a classic "Golden Ratio" for all sours...
Just made one of these after a while - what a delicious treat! I use aqua faba (the juice from a tin of chickpeas) instead of egg white, and I use an full ounce at that. I also like to do my bitters dashes on top of the foam, rather than shaken in with it. One part I missed was dry-shaking second, after a shake with ice (rather than the reverse) I'll give that a go next time!
Dry shaking last does yield better results. Works best with tin and tin two-piece shaker. Don't rinse between wet and dry shakes as tins need to remain cold from the first shake or you'll end up with a foaming but warm cocktail.