In a large bowl, whisk together the flour, salt, and sugar. Cut the butter into the flour using a fork or a pastry cutter. The butter should breakdown to the point where it resembles oat flakes and some may be the size of peas. Add the buttermilk and egg yolk to the mixture until the dough starts to form a ball. Dump the dough out onto a clean surface and form a ball. Place dough in plastic wrap and tightly wrap the dough. Alternately, you can place the dough back into the bowl and cover, refrigerate. Let dough cool for at least 30 minutes. Prepare apple pie filling while you wait.

Peel, core and dice apples into small, thin pieces. Place apples in a large bowl. Add orange juice, vanilla bean paste, and toss apples to evenly coat. In another bowl, whisk together flour, granulated sugar, light-brown sugar, cinnamon and nutmeg. Pour flour mixture over apple mixture and toss to evenly coat, set aside.

Place a rack near the center of the oven and preheat to 350 degrees F.

Divide the dough into 2 equal portions. Roll the dough out on a lightly floured surface into a 14 x 10 rectangle. Fold the dough over the rolling pin gently and transfer to a 13 x 9 baking pan. Toss the apple mixture and then pour it over the top of the prepared pie crust. If you've got access liquid pooling at the bottom, save the liquid, just pour the apples. Using a tablespoon, pour 2-3 tablespoons of the liquid over the apples. Roll the remaining dough into the same 14 x10 rectangle as before and transfer the dough over to the baking dish. Cover the apples and pinch the corners. Using a small pairing knife, prick small holes on the top crust.

In a small mixing bowl whisk egg white until foamy, brush egg white over top pie crust. In another small mixing bowl whisk together turbinado sugar with cinnamon. Sprinkle cinnamon sugar mixture evenly over the top of pie crust.

Let the apple pie bars bake for 50-60 minutes, or until golden brown. Remove from the oven and allow to cool for 30 minutes at room temperature. The bars are ready to serve after 30 minutes but they won't hold their shape. If you want perfect bars, place the baking pan in the refrigerator for 30 minutes. Slice once cooled (see note). Prepare the icing while they cool.

In a small bowl, combine the powdered sugar, 1 tablespoon milk, lemon juice, and vanilla extract. If the icing is thick and doesn't drip with a spoon, add the 1 teaspoon milk. Keep in mind if it's too thin it won't harden properly on the bars.

Drizzle icing over the apple pie bars.