In a large mixing bowl, combine the powdered milk confectioners' sugar, cocoa powder, pumpkin pie spice, cornstarch, and kosher salt.
Whisk all ingredients together until well blended.
Transfer the mixture to an airtight container. Store in a cool, dry place for up to 1 year.*
Measure ¼ cup of the hot cocoa mix into a glass mug.
Pour 1 cup of hot water or milk over the mix. Stir until the powder has dissolved and is smooth.
Top with mini marshmallows, if desired. (I highly recommend adding marshmallows, especially if using water, as they will add a silky velvet-like quality to the hot cocoa once melted.)
