Custom version:
2 oz rye bourbon
1 oz cognac
1 oz vermouth, split between sweet and dry
dash of aromatic bitters
1 maraschino cherry

Instructions

Pour bitters, liquors over ice in mixing glass. Stir and strain into martini glass, garnish with a maraschino cherry.

Dive deep: http://www.sfgate.com/cgi-bin/article.cgi?f=/c/a/2007/09/21/WI1O...

Joy of Drinking focuses on a simple bourbon or rye manhattan, and adds in peychaud's bitters to the mix, with a
1.5 oz rye, .25oz each sweet and dry vermouth, 2 dashes angostura, 1 dash peychauds.

http://thejoyofdrinking.blogspot.com/2011/12/essential-cocktails...

Aging

Manhattans age quite well. At charred hardwood chips for extra depth (or age in a barrel). I used a 3:2:1 ratio , using rye bourbon for additional spice (which mellows nicely), and I added additional sweet vermouth at serving, to taste.

further Variations

A Dry Manhattan is made with dry vermouth instead of sweet vermouth and served with a twist.
A Brandy Manhattan is made with brandy rather than rye.
A Ruby Manhattan is made with port rather than vermouth.
A Metropolitan is similar to a brandy manhattan, but with a 3-to-1 ratio of cognac or brandy to vermouth.