Having tried this two days ago, I wanted to do a back to back comparison using the PDT recipe: 1.5oz rye, only 1 oz Dubonnet, and more Cointreau .25 oz. And it is stirred, not shaken. I preferred this with less Dubonnet but it misses something by not being shaken as per the original recipe. And I don't just mean the "scummy head". Lost is some aroma, complexity, bitterness, and probably mouthfeel the Regal Shake purportedly can impart.

I had to try this intrigued by the description of a "scummy head" and that is exactly what one gets using the recommended Regal Shake! As to the cocktail itself, very tasty, but I may try the PDT variation next time which uses only 30ml of Dubonnet and ups the Cointreau to 7.5 (and which is stirred rather than shaken). Elsewhere I have read that the cocktail was in fact named after the American boxer Dave Deshler (no relation to the hotel Deshler) who fought from 1903-1917. While I can't comment on the veracity of the naming, I'm inclined to believe the story provided above in Difford's description.

Pretty good in general (without being a 5* top drawer go-to), but I have to agree with the popular voice - stirred is better than shaken.

Stirred and without peel this is a 'home on the range' which is a 5* beverage.

This is infinitely better stirred rather than shaken! Rules schmools.