
In a saucepan over medium-low heat, combine the water, brown sugar, mashed banana, cinnamon stick and salt. Bring to a low simmer and cook for 8 to 10 minutes until the sugar is dissolved and the mixture has reduced and thickened slightly. Remove from the heat and discard the cinnamon stick.
Transfer the syrup to a high-powered blender and puree until smooth. Stir in the vanilla extract, then transfer to a Mason jar to cool.
Fill a tall glass with ice. Add the banana bread syrup and espresso, and stir. Add the milk.
Meanwhile, in a small container with a tight-fitting lid, add 1 tablespoon of the banana bread syrup and 2 ounces of heavy cream. Seal with a lid and shake vigorously for several minutes until the mixture thickens and doubles in volume.
Pour the cold foam over the top of the latte in the glass. Garnish with a sprinkle of cinnamon and/or nutmeg, if desired.