Preheat the oven to 350°F. Grease a 9 x 5-inch loaf pan and line it with parchment paper, leaving enough overhang on the sides to easily lift out the finished loaf.

Make the cinnamon apples. In a medium bowl, toss the chopped apples with the brown sugar and cinnamon until evenly coated. Set aside.

Make the streusel. Combine the brown sugar, flour, cinnamon, and salt in a small bowl. Add the cold cubed butter and work it into the dry ingredients with your fingertips or a pastry cutter until large, buttery crumbs form. Refrigerate while you prepare the batter. A cold streusel will hold its shape better.

Mix the dry ingredients. In a medium bowl, whisk together the flour, cinnamon, nutmeg, baking powder, baking soda, and salt.

Cream the butter and sugars. In a large bowl, beat the softened butter, granulated sugar, and brown sugar on medium-high speed with a hand mixer until light and fluffy, about 2 minutes.

Add the wet ingredients. Beat in the eggs one at a time. Mix in the buttermilk, apple butter or applesauce, and vanilla until combined. The batter will look curdled after the addition of the buttermilk - this is normal.

Fold in half of the apples and spread half of the batter into the prepared pan. Spoon the remaining cinnamon apples and any juices evenly over the batter. Add the remaining batter on top and gently spread it to the edges of the pan.

Crumble the chilled streusel generously over the entire top of the loaf, keeping some larger pieces for extra crunch. Bake on the center rack for 60 to 75 minutes, or until a toothpick inserted into the center comes out almost clean with a few moist crumbs. If the streusel begins browning too quickly, loosely tent the loaf with a piece of foil.

Let the bread cool in the pan for about 15 minutes, then carefully lift it out and transfer it to a wire rack to cool completely.

Make the cinnamon glaze. Whisk together the powdered sugar, cinnamon, vanilla, and 2 tablespoons milk until smooth. Add additional milk, 1 teaspoon at a time, until the glaze is thick but pourable.

Finish the loaf. Once completely cooled, drizzle the cinnamon glaze generously over the streusel. Let the glaze set for 10 to 15 minutes before slicing.