Streusel Crumble Topping
Arrange oven rack in lower third of oven and preheat oven to 350ºF. Lightly grease 9×13 pan with butter. Optional to line with parchment to remove bars as one. Lightly greasing the pan first will prevent the parchment from sliding when spreading batter into the pan with a spatula.
Prepare topping: In a medium bowl, whisk together flour and sugars until well blended. Add vanilla and stir in with a fork; then work in with fingertips until well combined. Using fingers, a pastry blender, cut butter into flour mixture until it resembles a coarse meal with small pea-sized pieces of butter taking care not to overwork the dough. If it does not come together, add the water and remix with your hands. Cover and refrigerate until ready to use.
Beat together the butter and sugar until creamy. Add eggs one at a time. Combine well. Add apples, mix gently. Separately mix together the dry ingredients in another bowl. Add the dry ingredients to wet ingredients, slowly and mix gently. Scrape and pour batter into prepared pan and smooth the top with a rubber spatula. Sprinkle topping evenly over the top of the batter.
Bake bars in preheated oven until topping is golden brown, center appears set and a toothpick inserted in the center comes out with moist crumbs clinging to it (versus uncooked batter), about 30 to 35 minutes. To prevent over-browning of topping, cover with sheet of aluminum foil during last 5 minutes of baking.
Remove foil and carefully transfer to wire rack to cool completely, about 2 hours. Use the baking parchment overhang to lift the blondies from the pan and transfer to cutting board. Or cut directly in the pan. Cut to desired sizes.
Tips: Be sure to dice the apples small enough, at 1/2-inch, so that batter can easily be spread evenly into the pan with a rubber spatula.
To Cut Neat Squares: Chill the bars in the refrigerator for about 30 minutes or in the freezer for 10 to 15 minutes. Using a hard plastic knife will prevent them from sticking to your blade and give you a cleaner cut. This way, you will always have nice, clean-sliced bars, blondies and brownies. As soon as you cut the bars, store them in an airtight container because their exteriors (all that fudgy goodness) dries quickly when exposed to the air.