Heat the oven to 375°F with a center rack. Line or grease a standard 12-cup muffin pan and allow the oven to preheat thoroughly.
Whisk flour, baking powder, baking soda, salt and cinnamon in a large bowl. Toss optional walnuts or chips with 1 teaspoon of this mixture.
Mash ripe bananas mostly smooth, then measure exactly 1 1/4 cups. Mix together any thawed banana and released liquid before measuring.
Whisk banana, Greek yogurt and brown sugar until smooth. Whisk in eggs, oil and vanilla until glossy and uniform.
Pour wet ingredients into dry and fold gently until only a few flour streaks remain. Add optional walnuts or chips and fold just until the flour disappears.
Divide among 12 cups, about 1/3 cup each or roughly three-quarters full. Smooth sharp peaks and add coarse sugar if using.
Bake 18 to 22 minutes, until tops are rounded and golden, centers spring back, a tester shows no glossy batter and a representative center reaches about 200 to 205°F.
Cool in the pan 5 minutes, then transfer to a wire rack. Wait at least 15 minutes before opening a liner so the crumb can finish setting.