Mix the dry ingredients: In a medium bowl, whisk together the flour, baking powder, cinnamon, and salt until combined.
Mash and combine: In a larger bowl, mash the banana well with a fork until mostly smooth with a few tiny lumps. Add the egg and beat until blended. Whisk in the milk until smooth.
Combine wet and dry: Pour the dry ingredients into the wet. Whisk gently until just combined. A few lumps are fine — overmixing makes them tough.
Heat the pan: Place a nonstick pan over medium heat. Let it warm up for a full minute. Spray with cooking spray. Test: a drop of water should sizzle and evaporate immediately.
Pour the batter: Use a ⅓-cup measuring cup to pour batter into the pan. Cook 3 to 4 circles at a time, leaving space for spreading.
Wait for bubbles: Cook until edges look set, bubbles form and pop, and the bottom is golden brown, about 2 to 3 minutes.
Flip and finish: Slide a spatula under the pancake and flip confidently. Cook for another 1 to 2 minutes, until puffed and cooked through and golden on the bottom.
Keep warm: Set finished pancakes on a plate in a 200°F oven while cooking the rest, or serve immediately.