Add the pumpkin syrup, orange bitters and orange slice to a rocks glass and muddle.
Add the bourbon, 1 large ice cube and 1 splash of club soda, and stir several times with the cinnamon stick.
*Pumpkin syrup: Add 1 cup water and 3/4 cup light brown sugar to a saucepan and simmer until the sugar dissolves. Turn the heat to low and whisk in 3 tsp of pumpkin pie spice and 1/3 cup pumpkin purée. Simmer on low for 8 minutes, remove it from heat, and allow to cool. Strain through a fine-mesh strainer, add 1 tsp vanilla extract, and store the syrup in the refrigerator in a container with a tightly fitting lid for up to 1 month.