Preheat oven to 350°F and line the bottom of a 9 inch square pan with parchment paper and grease the sides. Melt the butter in a microwave safe bowl, then transfer to a mixing bowl. Add the brown sugar and whisk to combine then add the vanilla and eggs and whisk until incorporated. In another bowl, combine the dry ingredients; slowly add the dry ingredients to wet ingredients until combined.

Spread about half of the batter evenly into the bottom of the pan. Fold about 3/4 of the chopped apples into the remaining batter, then spread evenly into the pan. In another bowl, combine streusel ingredients and mix together until it forms a crumbly mixture. Stir in the remaining apples. Sprinkle the streusel over the top of the bars. Bake the bars for 33-35 minutes, then set aside to cool completely. Cut into bars and serve. Bars are best stored in the fridge.