Place 1 brown sugar cube on a cocktail napkin. Sprinkle 2 to 3 drops orange bitters and 2 to 3 drops Angostura bitters over sugar cube. (Napkin will soak up excess bitters.) Transfer cube to a 10-ounce old-fashioned glass. Add 1 orange slice and a few drops bourbon to glass. Mash sugar cube and orange slice, using a muddler, until sugar is almost dissolved. (Avoid mashing the rind; doing so will release a bitter flavor.) Add 3 tablespoons bourbon, and fill glass with ice cubes. Stir until well chilled.