Combine granulated sugar and water in a small saucepan and heat over medium until sugar is fully dissolved. Add orange zest, allspice berries, and the cinnamon stick. Simmer gently for 5 minutes to allow the aromatics to infuse; do not boil vigorously. Remove from heat, stir in 4 dashes of Angostura bitters, and let the mixture cool to room temperature, steeping for at least 20–30 minutes for stronger flavor. Strain through a fine mesh sieve or cheesecloth into a clean container, pressing lightly on solids to extract syrup. Discard solids. Refrigerate the syrup; it will keep for up to 2 weeks. Yield: about 3/4 cup (adjust quantities proportionally).

Cocktail

Chill glass and prepare ice

Chill an old-fashioned (rocks) glass by filling it with ice and setting it aside while you prepare the cocktail, or place the glass in the freezer for a few minutes. Prepare a single large ice cube or gather 2–3 large ice cubes.

Build the cocktail

Discard the chilling ice from the glass. Add 0.25 oz house bitters-forward syrup to a mixing glass or directly into the chilled old-fashioned glass. Add 2 oz bourbon and 0.75 oz Amaro Averna.

Add bitters and stir

Add 3 dashes of orange aromatic bitters to the mixing glass. If using a mixing glass, fill with ice and stir gently for 20–30 seconds until well chilled and properly diluted (about 25–30% dilution depending on ice). If built in the serving glass, add the large ice cube(s) and stir for 20–30 seconds to combine and chill.

Serve

Strain the stirred cocktail into the chilled old-fashioned glass over the large ice cube (or leave as built if already in the serving glass). Express the oil of an orange peel: hold the peel over the drink, skin side down, and give it a firm twist to spray oils onto the surface, then run the peel around the rim and drop it into the glass as garnish. Optionally add a second light spritz of orange bitters on top for extra aroma. Serve immediately.