Combine ingredients into a shaker with one cube of ice. Shake for 30 seconds. Open the shaker, add in a heap of ice, close the shaker and shake again. This time for 30 to 60 seconds. Alternately, shake with ice first to chill, strain into a chilled mixing glass or shaker and use milk frother for 10 to 15 seconds to build the foam. Strain into the cocktail glass whichever method you use. Strain the cocktail into a chilled coupe glass and garnish with a few drops of angostura bitters dropped on the top of the cocktail.