Preheat oven to 375°F. Line a 12-cup muffin pan or grease lightly.
Mash bananas in a large bowl. Whisk in milk, beaten egg, oil, vanilla, and sugar until smooth.
Stir in baking mix and cinnamon just until combined. Do not overmix.
Fold in nuts if using. Divide batter evenly among muffin cups. Sprinkle with coarse sugar if desired.
Bake 16–18 minutes, until golden and a toothpick comes out clean.
Cool in pan 5 minutes, then move muffins to a rack to finish cooling.