Preparation Steps

• Gather all ingredients.
• Preheat the oven to 350 degrees F (175 degrees C). Grease a 12x18-inch jellyroll pan.
• Beat yellow cake mix, water, eggs, and ½ 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.
• Meanwhile, make the crumb topping: Mix brown sugar, cinnamon, vanilla extract, and flour together in a large bowl. Rub in 1 ½ cups butter until the mixture forms medium crumbs.
• Remove cake from the oven, and press about ¾ of the crumb topping onto the top of the almost-baked cake. Reserve the remaining topping.
• Return cake to the oven, and bake an additional 10 to 12 minutes, until the crumb topping is firm and cake is cooked all the way through.
• Remove cake from the oven, and sprinkle with remaining crumb topping. Let cake cool to room temperature, and sprinkle with confectioners' sugar.