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.