1. Preheat and Prep:

• Preheat your oven to 350°F (175°C).

• Grease a 9×13-inch baking dish or line it with parchment paper for easy cleanup.

2. Make the Cake Batter:

• In a large mixing bowl, combine the cake mix, sour cream, eggs, vegetable oil, and vanilla extract.

• Beat until smooth and fully combined (about 1–2 minutes).

3. Pour into the Dish:

• Pour the batter evenly into the prepared baking dish.

• Smooth the top with a spatula.

4. Prepare the Crumble Topping:

• In a small bowl, stir together brown sugar, flour, and cinnamon.

• Add melted butter and mix with a fork until crumbly.

5. Top and Bake:

• Sprinkle the crumble evenly over the cake batter.

• Bake for 30–35 minutes, or until a toothpick inserted in the center comes out clean.

6. Cool and Serve:

• Let cool for at least 10 minutes before slicing.

• Serve warm or at room temperature with coffee, tea, or milk.