Crumble Coffecake

My husband has been on the hunt for the perfect coffee cake for two weeks now, and we are not there yet. This version is a not too sweet crumble cake, that is perfect for coffee drinkers who like that European level of sugar, but wasn't quite enough for my home front. It comes from Simply Julia, a cookbook for week nights. This can be doubled for a 9 x 13 cake. 1 Cup All-Purpose Flour 1/2 Cup Whole Wheat Flour 1 1/2 tsp Baking Powder 1/4 tsp Kosher Salt 1/4 Cup Vegetable Oil 1/2 Cup Granulated or Raw Sugar 1 Large Egg 1 tsp Vanilla Extract 3/4 Cup Buttermilk, well-shaken Crumb Topping Ingredients: 1/2 Cup All-Purpose Flour 1/2 Cup Whole Wheat Flour 1/3 Heaping Cup Granulated Raw Sugar 1 tsp Baking Powder 1 1/2 tsp Ground Cinnamon 1/2 tsp Kosher Salt 1/2 Cup (1 Stick) Unsalted Butter, at room temperature Powdered Sugar, for serving Prior to making, take the butter out to warm to room temperature. Prepare the 11 x 7 baking pan by spraying with cooking spray (as in Pam or Olive Oil spray). Preheat the oven to 350° Make the Cake Batter: To a medium bowl, add both flours, baking powder and salt. Whisk well to combine. In a separate bowl, whisk together the oil, sugar, eggs, vanilla, and buttermilk Stir the dry ingredients into the wet ingredients until just combined. Transfer the batter to the prepared baking dish. The batter will be thick, smooth with a rubber spatula. Make the Crumbs: Place the flours, sugar, baking powder, cinnamon, and salt in a large bowl and whisk well to combine Add the room temperature butter and mix to form large crumbs. The best way to do this is to use your own clean hands. Rub the mixture between your fingers to form crumbs about the size of marbles. It’s ok for them to be uneven. Evenly sprinkle the crumb mixture on top on the cake batter. (It will be a thick layer.) Bake about 35-40 minutes. Test for doneness using a toothpick or clean knife inserted all the way through the crumbs and into the cake. It will remove clean when the cake is done. If there is batter on the knife, bake for another 3-5 minutes and test again. Cool completely to room temperature. Once completely cooled, sprinkle with powdered sugar and cut into square to serve.