Prepare the Crumb Topping:
In a medium mixing bowl, combine the granulated sugar, flour, and cinnamon.
Add the cold, cubed butter to the mixture.
Use a fork or pastry blender to cut the butter into the dry ingredients until the mixture resembles coarse crumbs. Set aside.
Prepare the Cake Batter:
Preheat oven to 350°F (175°C). Grease and flour a 9×13 inch baking pan (or a 9-inch square baking pan).
In a large mixing bowl, combine the yellow cake mix, melted butter, sour cream, eggs, and vanilla extract.
Beat with an electric mixer on medium speed until well combined, about 2-3 minutes.
Assemble and Bake:
Pour the cake batter into the prepared baking pan and spread evenly.
Sprinkle the crumb topping evenly over the batter.
Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.
Cool and Serve:
