Preheat oven to 325°. Spray a 9x13 baking dish with cooking spray. Set aside. 

Whisk flour, sugar, baking powder, and salt together in a medium bowl. Set aside. 

Whisk egg, milk, oil, and vanilla in another bowl. Pour the wet ingredients into the dry ingredients and mix until just combined. 

Make the topping by mixing together flour, brown sugar, and cinnamon. Pour in melted butter and use a fork to combine. Mix until large clumps form. 

Pour the batter into the prepared pan. Spread evenly, it will seem like there isn’t enough batter, but there is. 

Sprinkle the crumb topping evenly over the entire surface of the batter. 

Bake for 20 to 25 minutes, or until a toothpick comes out clean, rotating the pan once during baking. 

Let cool completely and sieve powdered sugar over the top of the cake.