The Sidecar was a cocktail I got pretty fixated on for a while. It's still one that I revisit on occasion. There's some flexibility in the drink's core that allows it to accommodate adjustments made for mood, quality of ingredients on hand, etc. Traditionally it's made with equal parts brandy, triple sec and lemon though there are several worthwhile ratios that skew more heavily toward the brandy.
The Comet isn't too far removed from the Sidecar. Substitute grapefruit for the lemon, augment the triple sec with some Drambuie, keep the brandy and throw in some bitters. I came across the Comet in David Wondrich's online drinks column for Esquire. The original recipe calls for Van der Hum, a South African spirit Wondrich describes as "basically brandy, flavored with tangerines, herbs and such." The triple sec/Drambuie combo is recommended as an approximation.
Citrus all over the nose with grapefruit leading but followed by orange from the Cointreau and spices from the Drambuie and Angostura. I've never had Van der Hum but the pairing of Cointreau and Drambuie is something I'd like to explore further. Here they work well together with the Drambuie playing off of Cointreau's orange flavor while adding herbs and a subtle honey quality. It does this without getting in the way though. Those two ingredients are deployed with a light hand which allows the brandy (I used Armagnac) to anchor things while the grapefruit keeps the sweetness in check by adding a sour component. For the grapefruit, white is recommended here, it's less sweet than Ruby Red.
Comet - Eddie Clarke, Albany Club, London, 1950s
1 1/2 oz Cognac
1 oz Grapefruit Juice (white, if possible)
1/2 oz Van der Hum*
1 dash Angostura
Shake, strain, up
*kludge for Van der Hum = 1/4 oz Cointreau + 1/4 oz Drambuie