Place the banana, peanut butter, cooled coffee, and milk into the blender.
Blend on high for about 30 seconds until the mixture is smooth, creamy, and no banana chunks remain.
Stop the blender and taste the smoothie; add sweetener and a pinch of cinnamon or vanilla if desired.
Blend again for a few seconds to incorporate the added flavorings.
If the smoothie is too thick, add a splash more milk and blend briefly. If too thin, add a few ice cubes and blend until frosty and thickened.
Pour the smoothie into a glass, then optionally sprinkle with chopped nuts or a dash of cinnamon on top for garnish.
Enjoy immediately for the best texture and flavor — sip and savor the comforting, nutty, and coffee-infused goodness.