Preheat oven to 350 degrees F. Prepare a 9" x 5" bread pan by spraying liberally with non-stick cooking spray. In a small bowl, prepare cinnamon sugar mixture by combining brown sugar with 1 tsp cinnamon and mixing well until combined. Set aside.
Prepare apple mixture by combining diced apples with 2 tablespoons sugar and remaining teaspoon of cinnamon. Mix until well combined then set aside.
In a mixing bowl, combine butter and 2/3 cup sugar and beat with an electric mixer until light and fluffy, about 2-3 minutes. Beat in eggs, one at a time, until well blended. Add in vanilla and mix until well blended. Add flour and baking powder to the mixture and stir by hand until well blended. Add in milk and mix until combined and absorbed but do not over mix.
Scoop slightly more than half (approximately) of the bread mixture into prepared bread pan. Sprinkle half of the apple mixture over the batter. Sprinkle half of the cinnamon sugar mixture over the apples. using a knife, gently swirl the mixture into the batter*. Repeat process with remaining ingredients.
Bake in preheated oven for 50-60 minutes or until a toothpick inserted into the center comes out clean. Allow to cool in pan, on a cooling rack, for 10-15 minutes, then gently run a knife around the edges of the pan to help loosen the bread from the sides. Invert and remove from pan.
To make glaze, combine powdered sugar and milk and stir until smooth. Drizzle over bread once removed from the baking pan.