Spray an 8"x 8" baking dish with nonstick cooking spray. (For a thinner cake, use a 9"x 13" baking dish)
In a medium bowl combine 1 1/2 cups flour, sugar, baking powder and salt. Stir until combined.
In another bowl, combine egg, milk, oil and vanilla extract. Stir until it's all combined.
Combine flour mixture with milk mixture and mix well. Spread the batter into the prepared baking dish.
For the crumble topping:
In a medium bowl, combine brown sugar, flour and cinnamon. Mix it together well.
Then pour in melted butter and stir until combined and clumps begin to form.
Sprinkle those clumps (crumbs) all over the cake batter. If you are using an 8"x 8" baking dish - it will be thick.
Bake for about 40-50 minutes for an 8"x 8" dish. Bake for about 30-40 minutes for a 9"x 13" baking dish.
Use a toothpick to check for doneness (as oven times vary.) Check around the 30 minute mark.
Crumb topping should be golden brown and a toothpick inserted to the middle of the cake should come out clean.
Allow cake to cool then dust with powdered sugar and serve.
Video
Notes
Do not store this cake in the refrigerator. Like most homemade cakes, they can dry out and lose moisture when kept in the refrigerator. Store covered on the counter for up to 3 days. You can freeze this cake but it is likely it won't be as tender when defrosted.