Prep the pan: Heat oven to 375°F (190°C). Line a 24-cup mini muffin tin with liners or grease lightly.
Mix dry ingredients: In a medium bowl, whisk flour, sugar, baking powder, baking soda, and salt.
Mix wet ingredients: In a separate bowl, whisk egg, oil, yogurt, and vanilla until smooth.
Choose your flavor: For banana, stir the mashed banana into the wet mixture.
For blueberry, toss berries with 1 teaspoon flour to prevent sinking.
Combine: Add wet ingredients to dry. Stir gently just until no dry streaks remain. If making blueberry, fold berries in last. Do not overmix.
Fill the cups: Divide batter evenly—each cup about 3/4 full.
Sprinkle tops with coarse sugar if you like.
Bake: Bake 10–13 minutes, until tops are set and a toothpick comes out with a few moist crumbs.
Cool: Let muffins rest in the pan 5 minutes, then transfer to a rack to cool. Enjoy warm or at room temperature.