Preheat oven to 375. Grease 12 regular size muffin cups or 24 miniature muffin cups (or use paper liners). In bowl stir together 2 1/4 cups flour, baking soda, and salt. In smaller bowl mix egg, buttermilk, 1/2 cup applesauce, vanilla, and 1 1/2 cups brown sugar. Stir together until well blended, then add to the flour mixture and dump in your diced apples also. Mix all ingredients only until blended, then spoon into muffin tins.