Preheat oven to 350 degrees F. Spray a 9-by-5-inch loaf pan with nonstick spray and line with parchment paper, with ends hanging over the edge of pan. Spray parchment with nonstick spray.
Mix brown sugar and 1 teaspoon cinnamon together in a bowl; set aside. Toss apples with 2 tablespoons granulated sugar and the remaining teaspoon cinnamon in another bowl, set aside.
Beat butter and remaining 2/3 cup granulated sugar together with mixer until light and fluffy, about 5 minutes. With mixer and medium speed, add the eggs, 1 at a time; add vanilla extract.
Whisk flour, baking powder and salt together in another bowl, then add to creamed butter mixture and beat until blended. Mix 1/2 cup milk into batter until smooth.
Add 3/4 of the chopped apple mixture to batter.
Sprinkle half of the brown sugar/cinnamon mixture into batter; mix well.
Pour batter into the loaf pan.
Lightly pat remaining apples into batter, swirl remaining brown sugar mixture through apples using a knife.
Bake until toothpick inserted in the center of the loaf comes out clean, approximately 1 hour and 10 minutes. Watch bread and cover with foil during last 10 minutes of baking time to prevent over-browning if needed.
Let loaf rest in pan for 15 minutes, then remove from pan using edge of parchment and place on wire rack to cool completely
Mix powdered sugar with remaining 1 to 3 tablespoons milk, thinned to desired consistency.
Drizzle bread with glaze.
Substitute your favorite variety of baking apple for Granny Smith such as Jonagold, Honeycrisp, Braeburn or Cortland if desired
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