Preheat oven to 350°F. Grease a 9x5 or 8x4” loaf pan with shortening or butter and coat it with sugar. (You do this like you would grease and flour a pan. Grease it first, then add about 2 tablespoons sugar to the pan and move the pan side to side until the bottom and sides are coated with sugar. Do NOT substitute cooking spray for the butter. You can skip the sugaring and just use cooking spray, if you wish.)
Mash bananas in a large bowl with a fork or potato masher. Stir in oil, sugar, and eggs with wooden spoon or spatula. Set aside.
Add pumpkin spice, salt, baking soda, and vanilla and stir.
Mix in flour and sour milk and stir until just incorporated. Pour into prepared pan.
Bake for 45-60 minutes, or until a toothpick comes out clean in the middle of the crack. The edges will be a dark brown and there will be a nice crack down the center. (There is a wide range of cooking time because all ovens bake differently. Make sure to use a METAL pan, not glass.)
Cool completely before removing loaf from pan and frosting.
To make the frosting: whisk powdered sugar, pumpkin pie spice, and salt in a medium bowl. Whisk in vanilla and 1 tablespoon heavy whipping cream, adding more cream as needed for consistency (up to 3 tablespoons). Frost banana bread. Chill to set. Store in an airtight container for up to 3 days or slice and freeze for up to 1 month.