Preheat the Oven: Preheat your oven to 400ºF (200ºC). Line a regular 12-hole muffin pan with paper cupcake liners and set aside.
Melt Butter: Place butter in a small saucepan and melt over low heat, stirring occasionally. Alternatively, melt it in the microwave in 30-second bursts. Once melted, let it cool for 10-15 minutes until it's slightly warm to the touch, not hot.
Mix Dry Ingredients: In a large mixing bowl, whisk together the flour, baking powder, and baking soda. Whisk in the sugar and salt.
Add Wet Ingredients: Pour in the applesauce, oil, beaten egg, cooled melted butter, and vanilla. Fold and mix with a spatula until just combined, being careful not to overmix.
Fill Muffin Pan: Spoon the batter into the muffin liners, filling each cup about 3/4 full. For consistent size, use a small cookie scoop to place two scoops into each cup.
Bake: Bake for 15-18 minutes, or until the tops are golden and a toothpick inserted into the center comes out clean or with a few moist crumbs, not wet batter.
Cool: Allow the muffins to cool in the pan for 5 minutes, then transfer to a wire rack to cool completely. Enjoy!