Instructions:
- Preheat oven to 350°F (175°C). Grease the baking pan and line it with parchment paper, leaving an overhang for easy removal later.
- Prepare Apple Filling: Toss diced apples with lemon juice, sugar, and cinnamon. Set aside.
- Make the Streusel: In a medium bowl, combine streusel dry ingredients (flour, brown sugar, cinnamon, salt). Cut in the cold butter using your fingers or a pastry blender until the mixture resembles coarse crumbs. Set aside 1/3 of this mixture for the topping; leave the remaining 2/3 in the bowl.
- Mix Dry Cake Ingredients: Whisk together cake flour, baking powder, baking soda, salt, cinnamon, and nutmeg in a separate bowl.
- Cream Butter & Sugar: In the large bowl, beat the softened butter and granulated sugar until light and fluffy (about 3 minutes). Beat in eggs one at a time, followed by vanilla.
- Combine Batter: Alternately add the dry ingredients (in three additions) and the buttermilk (in two additions) to the creamed mixture, beginning and ending with the dry ingredients. Mix only until just combined.
- Assemble the Layers: Spread half of the cake batter into the prepared pan. Sprinkle the prepared diced apples evenly over the batter. Sprinkle the 2/3 portion of the streusel mixture directly over the apples. Gently press down slightly. Top with the remaining cake batter, spreading carefully. Finally, sprinkle the reserved 1/3 portion of the streusel evenly over the very top layer.
- Bake for 50–60 minutes. The cake is done when a skewer inserted into the center (avoiding apple chunks) comes out clean, and the top is golden brown.
- Cool: Let the cake cool in the pan on a wire rack for at least 30 minutes before using the parchment overhang to lift it out for complete cooling.