Make the streusel topping first – Combine 4 tbsp melted butter, 1/3 cups flour & 1/2 cup sugar together, until crumbly mixture forms. Refrigerate until ready to use.

Butter the inside of a 9"x12" baking dish with 2 tbsp butter. Now add 1 tbsp of flour and shake the dish around covering the inside with flour. Alternatively, you can spray the inside with a nonstick spray. Set aside.

Preheat oven to 350F with the baking rack in the middle.

Prepare the apples: Peel, core, and dice 3-6 apples into 1/2 inch cubes, then add 3 tbsp of flour and toss together to coat the apples lightly.

Whip 1/2 cup softened butter with 1 cup of sugar for about 2-3 minutes.

Meanwhile, combine the wet ingredients: In a cup whisk 1/2 cup buttermilk, 2 room temperature eggs and 2 tsp of the vanilla extract. Set aside. 

Combine the dry ingredients: In a bowl sift or whisk together 1/2 teaspoon salt, 1 1/2 teaspoon baking powder and 2 cups all-purpose flour.

Combine into the batter: Alternate adding the dry and wet ingredients into the whipped butter, by adding them in thirds. Do not overmix or the cake will be tough.

Next, fold the apples into the batter. Transfer the apple cake batter into the prepared baking dish and spread into an even layer. Sprinkle the top with the streusel topping by breaking it into smaller pieces with your hands.

Bake at 350F for 45 minutes or until a toothpick inserted in the center comes out clean. Allow to cool before slicing and serving.