Chill a rocks glass in the freezer for a few minutes or fill it with ice while you prepare the cocktail. A cold glass keeps the drink crisp longer.
Add the simple syrup, Angostura bitters, and orange bitters to a mixing glass. The mixture should turn a light amber color.
Pour in the Japanese whisky. Measure carefully to keep the cocktail balanced.
Fill the mixing glass with fresh ice and stir gently for about 10–15 seconds. The outside of the glass should become very cold and the drink should look clear.
Place one large clear ice cube into the chilled rocks glass. Large ice melts slowly and reduces dilution.
Strain the cocktail over the ice cube. The finished drink should appear bright and deep golden amber.
Twist the orange peel over the glass to release its oils, then drop it into the cocktail. Serve immediately.