Preheat your oven to 350°F. Spray a 9" x 13" glass or metal baking pan with cooking spray or line it with parchment paper for easy cleanup. Set aside while you mix the ingredients.

In a medium bowl, combine the brown sugar, all-purpose flour, and ground cinnamon. Cut in the cold, cubed butter using a pastry blender or two forks until the mixture resembles coarse crumbs. This will be your cinnamon mixture for both the center and the top of the cake. Pop the bowl in the fridge until you're ready to layer it in.

In a large mixing bowl, beat together the box of yellow cake mix, instant vanilla pudding mix, vegetable oil, water, vanilla extract, and butter flavoring with an electric mixer, until smooth.

Pour half of the cake batter into your prepared pan, spreading it out into a thin, even layer. It doesn’t have to be perfect—just enough to cover the bottom.

Sprinkle half of the cinnamon streusel mixture evenly over the cake batter. This layer gives the center of the cake its cozy, cinnamon roll-like swirl.

Dollop the rest of the batter over the cinnamon layer. Use a spatula to gently spread it out—it’s okay if some streusel peeks through. Finish by sprinkling the remaining brown sugar mixture over the top of the cake batter.

Bake the coffee cake for 40 to 45 minutes, or until a toothpick inserted in the center of the cake comes out clean. The top of the cake should be golden with a slightly crisp streusel topping. Let the entire cake cool completely in the pan before icing.

In a small bowl, whisk together the powdered sugar, vanilla extract, and 1-2 Tablespoons of milk (or non-dairy milk if needed). If the glaze is too thick, add a little more milk, a teaspoon at a time. Drizzle the classic vanilla glaze over the crumble topping.