Make the frosting: Beat butter until creamy with a hand or a stand mixer. Mix in salt. Slowly add in powdered sugar until mixture is crumbly. (It will be very crumbly if using a hand mixer, more smooth if using a stand mixer.) Add vanilla, pumpkin spice and 2 tablespoons heavy whipping cream. Mix until the frosting comes together and becomes smooth, adding remaining tablespoon of heavy whipping cream as needed for consistency. Frost.