Mix the butter, sugar, and vanilla until light and fluffy.
Add the eggs one at a time.
In a separate bowl, whisk together the flour, baking powder, and salt.
Add the flour mixture and milk to the butter mixture and mix until well incorporated.
In a separate bowl, mix together the diced apple, brown sugar, and 1 ½ teaspoons cinnamon.
Grease and flour a loaf pan.
Add half of the batter to the pan, then half of the apple mixture and swirl with a toothpick.
Add the remaining batter, then the remaining apple mixture and swirl.
Sprinkle cinnamon and brown sugar over the top.
Bake at 350°F for 45-55 minutes until a toothpick comes out clean.
Make the glaze by mixing the powdered sugar, ½ teaspoon cinnamon, and 2 tablespoons milk.
