Crack the cinnamon stick and add it into a small saucepan, along with the coriander seeds, and toast over medium-low heat until the spices become aromatic (~2 minutes).
Add water, brown sugar, and nutmeg, and stir to combine. Adjust the heat to medium, and bring to a simmer. Cook for 5 minutes, stirring occasionally. Remove from heat and let cool to room temperature.
Strain through a fine mesh strainer, cover, and store in the refrigerator until ready to use.
For the cocktail
Add sweet potato pie spice syrup, bitters, sweet potato liqueur, and rye whiskey into a mixing glass. Fill mixing glass with ice and stir for 30 seconds. Strain into an old fashioned glass over one large ice cube. Garnish with a twist of orange, and serve!