Place the room temperature butter and brown sugar in a large bowl. Beat the two ingredients together until creamy. Add the room temperature egg to the bowl and beat it into the butter /sugar mixture until it's well combined.Place the flour, baking powder, baking soda and salt in a large bowl. Stir the ingredients together to combine them. Add the dry ingredients and applesauce to the creamed mixture alternately. Add the dry ingredients in three batches and the applesauce in 2 batches. Example - Add one batch of dry ingredients to the creamed mixture, beat until JUST incorporated - add one batch of applesauce, beat until just incorporated. Add another batch of flour, beat until the flour is "just" incorporated. Add the last batch of applesauce, beat until just incorporated, add the last batch of dry ingredients, beat until just incorporated. Stir the oats and chocolate chips into the batter. Place muffin liners into your muffin tin. Fill the paper liners 3/4 full with batter. Bake at 350 degrees for 25 minutes or until a toothpick inserted in the center comes out clean.