Step 1

Preheat oven to 350 degrees F (175 degrees C). Grease a 12x18-inch jellyroll pan.

Step 2

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.

Step 3

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.

Step 4

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.

Step 5

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.

Step 6

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.