Instructions:
- Preheat oven to 350°F (175°C). Grease and flour a 9-inch springform or Bundt pan thoroughly.
- Prepare the Apples: Toss the diced apples with lemon juice, 1/4 cup brown sugar, and 1 teaspoon cinnamon. Set aside.
- Make the Cinnamon Swirl Mixture: In a small bowl, cream together 4 tablespoons softened butter, 1/2 cup granulated sugar, and 1 tablespoon cinnamon until well combined and sandy. Set aside.
- Combine Dry Ingredients: Whisk the flour, baking powder, and salt in a medium bowl.
- Cream Butter & Sugar: Beat the 1 cup of softened butter and 1 cup of sugar until light and fluffy (about 3 minutes).
- Add Eggs and Vanilla: Beat in the eggs one at a time, followed by the vanilla extract.
- Alternate Wet & Dry: Gradually add the dry ingredients to the wet mixture, alternating with the buttermilk (starting and ending with dry ingredients). Mix only until just combined to avoid overdeveloping the gluten.
- Layer the Cake: Spoon half of the cake batter into the prepared pan. Sprinkle half of the cinnamon swirl mixture and half of the spiced apples evenly over the batter. Top with the remaining cake batter, then scatter the remaining cinnamon swirl and apples on top.
- Bake for 55 to 65 minutes, or until a wooden skewer inserted near the center (avoiding large apple pockets) comes out clean or with moist crumbs attached. If browning too fast, loosely tent with foil.
- Cool the cake in the pan for 15 minutes before inverting it onto a wire rack to cool completely.
- Make the Glaze (Optional): Whisk powdered sugar, vanilla, and 2-3 tablespoons of milk until a smooth, drizzly consistency is achieved. Drizzle over the fully cooled cake before serving.