Preheat Your Oven Heat oven to 350°F (175°C). Line a 9×9-inch square baking dish with parchment paper or lightly grease it with butter or spray.
Make the Cake Batter In a large bowl, cream the butter and sugar until light and fluffy (about 2 minutes). Beat in eggs one at a time, then stir in vanilla.In another bowl, whisk together flour, baking powder, baking soda, and salt.Add half of the dry ingredients to the wet mixture, then fold in sour cream, followed by the remaining dry ingredients.Gently fold in diced apples — they’ll keep the cake moist and flavorful.
Add the Cinnamon Sugar Layer Mix brown sugar and cinnamon in a small bowl.Spread half the batter into your prepared pan, sprinkle the cinnamon sugar evenly over it, then top with the remaining batter. Smooth it gently with a spatula.
Make the Crumb Topping Combine flour, sugars, and cinnamon in a bowl. Drizzle in the melted butter and stir until clumps form.Sprinkle the crumbs evenly across the top of the batter.
Bake Bake for 35–40 minutes, or until a toothpick inserted in the center comes out clean.(Start checking around 30 minutes; ovens vary.)
Cool & Serve Let the cake rest in the pan for 15 minutes, then slice. Serve warm or at room temperature.Optional: dust with powdered sugar or drizzle with a light vanilla glaze.