Whoa Nellie!

1.25oz Bonded Rye Whiskey
.75oz Dark Rum
.75oz Cointreau
4 Dashes Angostura Bitters
.5oz Lemon Juice
.5oz Grapefruit Juice
.5oz Simple Syrup

Shake all of the ingredients over ice and double strain into a martini glass. 

The rest of the ingredients provide a sweet, tart & boozy background for the bitters to shine.