Place brown sugar cubes on a cocktail napkin. Sprinkle orange bitters and Angostura bitters over sugar cubes. (Napkin will soak up excess bitters.) Transfer cubes to a 10-oz. old-fashioned glass. Add orange slice and a few drops bourbon to glass. Mash sugar cubes and orange slice, using a muddler, until sugar is almost dissolved. (Avoid mashing the rind; doing so will release a bitter flavor.) Add 1 1/2 to 2 oz. bourbon, and fill glass with ice cubes. Stir until well chilled. Add more bourbon, if desired.