Add 5ml of powdered sugar and 2 dashes of Angostura bitters to an Old-Fashioned glass.
Add 5ml of water and stir until the sugar is fully dissolved.
Add a large ice cube or several ice cubes to the glass.
Pour in 45ml of aged rum and stir gently for about 10 seconds to combine and chill.
Express a lime peel over the glass to release its oils, then place it in the drink as garnish.
Carefully float 15ml of 151-proof overproof rum on top by pouring it slowly over the back of a bar spoon.
Serve immediately without further stirring.
Floating the overproof rum on top rather than stirring it in creates a dramatic first sip of high-proof intensity that gradually integrates as the drink opens up - resist the urge to stir it in. Using a demerara syrup in place of powdered sugar will add a deeper, more molasses-forward sweetness that complements aged rum beautifully.
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