Prep the glass. Chill a rocks or coupe glass in the freezer for 5 minutes so the cocktail stays frosty.
Combine ingredients. Add the banana, rum, banana liqueur, cream, milk, simple syrup, vanilla, and salt to a blender.
Blend with ice. Add 1 cup of ice and blend until smooth and frothy, about 15–20 seconds.
For a lighter texture, blend briefly; for thicker, blend longer.
Taste and adjust. If you want it sweeter or stronger, add a splash more syrup or rum and pulse once.
Serve. Pour into the chilled glass. Garnish with a banana slice and a light dusting of nutmeg or cinnamon.
No blender option. Muddle the banana in a shaker, add liquids and ice, then shake hard for 20 seconds. Double strain into your glass.