First, preheat the oven to 350-degrees, and grease/spray 9x13" pan.
Mix together and set aside:
2 cups sifted all-purpose flour
2 teaspoons baking powder
1/2 teaspoon salt
1/2 teaspoon ground cinnamon
Mix together in another bowl:
1/2 cup (1 stick) butter, melted
1 cup packed brown sugar
1 cup white granulated sugar
2 teaspoons vanilla extract
Now, blend the flour mixture into the sugar mixture until just combined, then add:
2 cups peeled and chopped apples (some fine pieces, and some not-so-fine) and 1/2 or 1 cup finely chopped walnuts OR pecans (optional)
Spread the above combined mixture evenly into a greased 9x13" pan.
*Before baking, sprinkle top of batter with a mixture of:
2 tablespoons white sugar
2 teaspoons ground cinnamon
Bake for 25 to 30 minutes in preheated 350-degree oven; finished bars should spring back when lightly touched. Cool in the pan, and then cut into squares.
*If you don't care for the topping of cinnamon/sugar, you could omit that step and later drizzle a vanilla or "browned butter"** frosting glaze over them once they have cooled.
** "Browned butter" glaze is made as follows:
Heat ½ stick butter in pan until golden brown in color (stirring all the while). Quickly add about two Tbsp. milk and then some SIFTED powdered sugar till you have the amount of frosting you need. It may be necessary to add tiny amounts of more milk to get a smooth texture. Quickly, drizzle this over the bars before it gets to firm to 'drizzle'.