Make the marshmallow cold foam by adding the heavy cream, toasted marshmallow syrup, and whole milk to a small glass or jar and frothing with a milk frother for 10-20 seconds or until foamy. Set aside.
Add ice to a tall glass, then pour in the pumpkin spice syrup, followed by the espresso. Top with milk of choice, then stir.
Top the pumpkin spice latte with the marshmallow cold foam, then drink up!