Prep the pan and oven. Heat the oven to 350°F (175°C).
Line a 12-cup muffin tin with paper liners or grease lightly. This helps the muffins release cleanly and keeps the bottoms tender.
Mix dry ingredients. In a large bowl, whisk together the flour, baking powder, baking soda, cinnamon, and salt. Stir to evenly distribute the leaveners so you get a consistent rise.
Combine wet ingredients. In a separate bowl or large measuring cup, whisk the egg, yogurt, applesauce, oil, sugar, and vanilla until smooth.
The mixture should look creamy and lump-free.
Bring it together. Pour the wet ingredients into the dry. Add the grated apple. Gently fold with a spatula until just combined.
Stop mixing as soon as you no longer see dry streaks. A few small lumps are fine.
Fill the cups. Divide the batter evenly among the muffin cups, about 3/4 full. If desired, sprinkle a tiny pinch of cinnamon on top for extra aroma.
Bake. Place the pan on the center rack and bake for 15–18 minutes, or until the tops are set and a toothpick comes out clean or with a few moist crumbs. Avoid overbaking to keep them soft for toddlers.
Cool. Let the muffins rest in the pan for 5 minutes, then move them to a wire rack to cool completely.
This prevents soggy bottoms and helps the crumb set.
Serve. Offer plain, or with a thin smear of yogurt, nut butter, or cream cheese for extra protein and healthy fats.