1-First Step: Prepare the Cake Batter and Pans Preheat oven to 350°F (175°C). Grease and flour three 8-inch round pans or line with parchment. In a bowl, whisk together 2 1/2 cups flour, 2 tsp baking powder, and 1/2 tsp salt. Set aside dry mix. In a large bowl, beat 1 cup softened unsalted butter and 2 cups sugar until light and fluffy, about 3 minutes. Add 4 eggs one at a time, beating well after each. Mix in 1 tbsp vanilla extract. Alternate adding dry ingredients and 1 cup milk to the wet mix, starting and ending with dry. Begin with half the flour mix, then half the milk, repeat. Stir gently until just combined, avoiding overmix. Divide batter evenly among pans, about 1 1/2 cups each. Smooth tops with a spatula.

2-Second Step: Bake the Cake Layers Bake cakes at 350°F for 25-30 minutes. Check doneness with a toothpick inserted in center; it should come out clean or with moist crumbs. Rotate pans halfway for even baking if your oven heats unevenly. Cool in pans 10 minutes, then turn onto wire racks to cool fully, at least 1 hour. Level tops with a serrated knife if domed for flat stacking.

3-Third Step: Make the Apple Filling While cakes cool, prepare filling. Peel and thinly slice 6 cups Granny Smith apples. In a saucepan over medium heat, combine apples, 1/2 cup brown sugar, 2 tsp cinnamon, 1/4 tsp nutmeg, and 2 tbsp lemon juice. Cook 5 minutes until apples soften. Mix 2 tbsp cornstarch with 2 tbsp water to make slurry; stir in. Simmer 5 more minutes until thickened. Cool completely, about 30 minutes. This prevents soggy layers.

4-Fourth Step: Prepare the Streusel For streusel, mix 1 cup flour, 1/2 cup brown sugar, 1 tsp cinnamon, and 1/2 cup cold butter cut into cubes. Rub with fingers or pulse in food processor until crumbly pea-sized bits form. Spread on a baking sheet and bake at 350°F for 10 minutes, stirring once. Cool fully; it crisps as it cools.

5-Fifth Step: Whip Up the Frosting Beat 1 cup softened butter until creamy. Gradually add 4 cups powdered sugar, 2 tsp vanilla, and 2-4 tbsp milk until fluffy and spreadable. Adjust milk for piping consistency if desired. Set aside.

6-Final Step: Assemble and Chill Place first cake layer on serving plate. Spread half apple filling, then sprinkle 1/3 streusel. Top with second cake, remaining filling, half remaining streusel. Add third cake, frost top and sides smoothly. Press last streusel around sides for decoration. Chill 30 minutes to set. Slice with sharp knife. Serve at room temp for best flavor.