Preheat oven and prep pan. Preheat your oven to 325° F and prepare a 9x13 baking dish by spraying with cooking spray, then set aside.
Make the crumb topping. I like to make the crumb topping first so that it's ready to go after the cake ingredients are mixed. Simply add flour, brown sugar, and cinnamon to a large bowl and mix to combine. Then pour in melted sticks of butter. Using a fork, mix until the ingredients are wet but form large clumps, or "crumbs", then set aside.
Mix the cake batter: In a medium bowl, whisk together flour, granulated sugar, baking powder, and salt. Then in a separate bowl, combine the egg, milk, oil, and vanilla and whisk until completely combined.
Then pour the wet ingredients into the dry ingredients bowl, and mix until they are just combined.
Pour batter and add crumbs. Pour the cake batter into the sprayed baking pan. You can also line it with parchment paper. Spread until it fills the pan evenly. (It will seem as if there is not enough batter, but the cake will rise).
Sprinkle the crumb topping evenly over the entire surface of the batter. Press down very lightly to make sure the bottom of the crumb topping is set in the cake batter.
Bake and serve: Bake for 20-25 minutes, or until an inserted toothpick comes out clean. Rotate the pan once halfway through baking. When the cake is done, remove it from the oven and let cool completely. Then sprinkle with powdered sugar over the top of the cake before serving.