Mix buckwheat flour, cornmeal, baking powder, baking soda, and salt.
In a large bowl, combine mashed banana, maple syrup, vinegar, and milk. Add flour mixture, stirring just enough to remove any lumps and make a pourable batter. Add a bit more milk if batter seems too thick.
Preheat a non-stick skillet or griddle, then spray lightly with vegetable oil. Pour small amounts of batter onto the heated surface and cook until tops bubble. Turn carefully with a spatula and cook the second sides until browned, about 1 minute. Serve immediately.