Preheat oven to 350 degrees. In a bowl, whisk together baking mix, sugar, and cinnamon; use fingers to work butter completely into mixture. Stir in banana, milk, and eggs until well combined. Divide evenly among 12 paper-lined standard muffin cups, filling each three-quarters full. Sprinkle generously with sugar.
Bake muffins until tops spring back when lightly touched, about 22 minutes. Let cool in pan on a wire rack 5 minutes. Remove from pan; let cool completely on rack.