Beat yellow cake mix, water, eggs, and 1/2 cup softened butter together with electric mixer until thoroughly blended (about 2 min on medium). Pour cake batter into jellyroll pan.
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Bake in preheated oven until edges are light brown but middle of cake is slightly gooey, 15 to 17 minutes. While cake is baking, mix brown sugar, cinnamon, vanilla extract, butter, and flour together until mixture forms medium crumbs. Remove cake and press about 3/4 of crumb topping onto the top of the almost-baked cake. Save remaining topping.
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