1. Preheat oven to 350°F (176°C) and line the bottom of a 9 inch square pan with parchment paper and grease the sides.
2. Melt the butter in a microwave safe bowl, then transfer to a mixing bowl.
3. Add the brown sugar and whisk to combine.
4. Add the vanilla and eggs and whisk until incorporated
5. In another bowl, combine the dry ingredients.
6. Slowly add the dry ingredients to wet ingredients until combined.
7. Spread about half of the batter evenly into the bottom of the pan.
8. Fold about 3/4 of the chopped apples into the remaining batter, then spread evenly into the pan.
9. In another bowl, combine streusel ingredients and mix together until it forms a crumbly mixture. Stir in the remaining apples.
10. Sprinkle the streusel over the top of the bars.
11. Bake the bars for 33-35 minutes, then set aside to cool completely.
12. Cut into bars and serve. Bars are best stored in the fridge.