Preheat your oven to 350 degrees. Grease 2 standard loaf pans and set aside.
In a bowl, combine the flour, baking powder, baking soda, salt, nutmeg, cinnamon, and allspice. Stir together and set aside.
In your mixing bowl, beat together the butter and sugar until fluffy. Add in the eggs, one at a time.
Mix half of the flour mixture into the creamy mixture until just incorporated. Next, mix in the buttermilk and then finish with the remaining flour mixture, until just combined. Stir in your diced apples.
Spread the mixture into the loaf pans and bake for 1 hour or until they test done with a toothpick. Allow the loaves to cool a couple minutes and then remove them from a pan to a cooling rack. Prepare the topping.
Measure out the vanilla and set it aside. Combine the remaining ingredients in a saucepan and melt it over medium heat, stirring with a whisk. Bring it to a boil, and let it boil for 3 minutes. Remove from heat and brush over the tops of the loaves.