Whirl a third of the ice cream, pumpkin, milk, and bourbon at a time in a blender until smooth, adding sugar and spices to last batch. Pour into a large bowl and stir to combine.
Ladle shakes into 8 tall glasses and top each with a dollop of whipped cream if you like and a little nutmeg. Serve with straws.
Make ahead: Up to 1 hour through step 1, freeze airtight; stir before ladling into glasses.
Note: Nutritional analysis is per serving.