A big sheet cake that starts its life as a boxed cake mix gets a topping made of tons of buttery, crumbly streusel.
- 1 (18.25 ounce) package yellow cake mix
- ⅔ cup water
- 3 large eggs eggs
- ½ cup butter, softened
- 1 ⅓ cups packed brown sugar
- 1 tablespoon ground cinnamon, or to taste
- 1 tablespoon vanilla extract
- 1 ½ cups butter
- 4 ½ cups all-purpose flour
- 1 tablespoon confectioners' sugar for dusting
- Preheat oven to 350 degrees F (175 degrees C). Grease a 12x18-inch jellyroll pan.
- Beat the yellow cake mix, water, eggs, and 1/2 cup softened butter together with an electric mixer in a mixing bowl until thoroughly blended, about 2 minutes on medium speed. Pour the cake batter into the prepared jellyroll pan.
- Bake in the preheated oven until the edges of the cake are light brown but the middle of the cake is still slightly gooey, 15 to 17 minutes.
- While the cake is baking, mix together the brown sugar, cinnamon, vanilla extract, butter, and flour together in a large bowl until the mixture forms medium crumbs. Remove the cake from the oven, and press about 3/4 of the crumb topping onto the top of the almost-baked cake. Reserve the remaining topping.
- Return the cake to the oven, and bake an additional 10 to 12 minutes, until the crumb topping is firm and the cake is cooked all the way through.
- Remove the cake from the oven, and sprinkle with the remaining crumb topping. Let the cake cool to room temperature, and sprinkle with confectioners' sugar.