Preheat the Oven: Set your oven to 350°F (175°C). Grease a 9x9-inch baking pan with butter or cooking spray.
Make the Streusel: In a small bowl, combine brown sugar, flour, and cinnamon. Stir in melted butter until coarse crumbs form. Add nuts if using, then set aside.
Mix Dry Ingredients: In a midsize bowl, whisk together flour, baking powder, baking soda, and salt.
Cream Butter and Sugar: In a large bowl, beat softened butter and granulated sugar with an electric mixer for 2 minutes until light and fluffy.
Add Wet Ingredients: Beat in eggs one at a time, then add vanilla extract. Mix in sour cream until smooth.
Combine Batter: Gradually add half the dry mixture to the wet, stir, then add the rest. Fold in chopped apples gently—don’t overmix!
Assemble: Spread the batter evenly in the prepared pan. Sprinkle the streusel topping over the entire surface.
Bake: Cook for 35–40 minutes, until a toothpick poked in the middle comes out clean. If the top browns too fast, tent with foil after 25 minutes.
Cool and Serve: Let the cake rest in the pan for 15 minutes before slicing. Enjoy warm or at room temp!