1.5 parts Bacardi Black
0.5 part Fresh Lime Juice
0.5 part Simple Syrup
1 part Dark Beer
2 Dashes Orange Bitters (or your favourite aromatic bitters)

Combine, Bacardi Black, Lime Juice, Simple Syrup and bitters in a mixing glass and shake. Next? Add the Dark Beer and strain into an ice filled rocks glass. Garnish with an Orange Slice.