Preheat and prep: Heat the oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners or grease lightly.
Mix the wet ingredients: In a large bowl, whisk yogurt, eggs, mashed banana, maple syrup (or honey), oil, and vanilla until smooth.
Whisk the dry ingredients: On top of the wet mixture, add flour, baking powder, baking soda, cinnamon, and salt. Gently whisk just until most of the flour is moistened.
Do not overmix.
Fold in berries: Add blueberries and fold with a spatula. If using frozen berries, toss them in 1 teaspoon flour first to reduce color bleed.
Portion the batter: Divide evenly among the muffin cups. Fill almost to the top for domed muffins.
Sprinkle with oats if using.
Bake: Bake 16–20 minutes, or until the tops are set and a toothpick comes out clean or with a few moist crumbs. Mini muffins take 12–14 minutes.
Cool: Let muffins rest in the pan for 5 minutes, then transfer to a rack to cool completely. This helps set the crumb for a soft, non-gummy texture.