Preheat Oven and Prep Tin:
Set oven to 375°F (190°C). Prepare a 12-cup muffin tin with paper liners or a light greasing.Whisk Dry Ingredients:
In a large bowl, whisk together flour, baking powder, baking soda, salt, cinnamon, and nutmeg until evenly distributed.Mix Wet Ingredients:
Combine melted butter with granulated and brown sugars, stirring until smooth. Add eggs, applesauce, and vanilla, mixing until fully homogeneous.Combine Wet and Dry:
Pour the wet mixture into the dry ingredients. Fold gently with minimal strokes, just until no dry streaks remain—do not overmix.Fill Muffin Cups:
Divide batter evenly among the muffin cups, filling each roughly two-thirds full.Bake Muffins:
Bake for 18-22 minutes, until a toothpick inserted into a muffin's center emerges clean.Cool and Glaze:
Let muffins cool in the tin for 5 minutes, then move to a wire rack to cool fully. If desired, brush warm muffins with melted butter and dust with a cinnamon-sugar mix.