Really Good Sour Cream Coffeecake with Serious Amounts of Crumb Topping

That's one descriptive title, isn't it?

I've loved coffeecake all my life, but only certain types. No berries, thanks. Yes to apple or rhubarb. Sour cream makes it so nice and moist. But always, always, it's really about the crumb topping.

A while back I found a great rhubarb coffee cake recipe that fully addressed the crumb topping issue. But I can't leave well enough alone so I tinkered with it a bit. When I went to make it today rhubarb wasn't in season and I couldn't find any frozen rhubarb. Hence, this amalgamation/refinement.

Inspiration #1: Big Crumb Coffee Cake on the Smitten Kitchen blog. Full endorsement of this recipe as-is if you don't have ingredients for my variation on her crumb topping. (She just uses flour--I added additional nutrition and flavor with ground flaxseed, walnuts, and oatmeal.)

Inspiration #2: Favorite Sour Cream Coffeecake from King Arthur Flour

Also--I don't understand why anyone ever makes a 9x9 coffeecake. Why go to all that trouble for just a couple of days of coffeecake? (At my house, at least, where my bike-racing husband runs a nuclear power plant he calls a metabolism, a 9x9 coffeecake doesn't last long.) So I always double coffeecake recipes. I've learned that this more than doubles baking time so keep an eye on this and test with a toothpick. The recipes linked above are for a 9x9 pan.

Cake

1/2 cup unsalted butter
1/2 cup canola oil or another bland vegetable oil
2 cups white sugar
4 large eggs
4 cups whole wheat pastry flour or unbleached all-purpose flour*
2 teaspoons baking powder
1/2 teaspoon baking soda
3/4 teaspoon salt
2 cups sour cream or yogurt**

* Whole wheat pastry flour is my preferred baking flour. It behaves like all-purpose white flour but gives you more nutritional bounce for the ounce.

** Feel free to use reduced-fat sour cream/yogurt, and vanilla yogurt would add a nice touch. If you like fruit in your coffeecake I suppose you could use fruit yogurt but I won't vouch for it.

Crumb Topping 

2/3 cup dark brown sugar
2/3 cup granulated sugar
2 teaspoons ground cinnamon
3/4 teaspoon ground ginger
1/2 t. nutmeg
1/4 t. cloves
1/4 teaspoon salt
1 cup unsalted butter, melted (you need this much to make it clump deliciously)
1 c. ground flaxseed
1 c. walnuts, finely chopped or whizzed in blender
1 c. oatmeal (quick or regular--doesn't matter--can substitute oat bran too)
1/2 c. whole wheat flour or unbleached all-purpose flour

Cream together the butter and oil, sugar, and eggs. In a separate bowl, mix together the flour, baking powder, baking soda and salt. Add the flour mixture to the butter mixture alternately with the sour cream or yogurt, stirring after each addition.

(Honestly, if you're like me you just sprinkle the flour and leavening agents in rather than going to the whole separate-bowl trouble. Just don't dump the baking powder or soda all in a clump; you can end up with bitter white spots in your final product if it doesn't get mixed in. Sprinkle it over the surface of the batter.)

To make crumb topping, mix sugars, spices, and salt into melted butter, then stir in flaxseed, walnuts, oatmeal, and flour. Leave it pressed together into the sides and bottom of the bowl.

Grease and flour a 9x13 baking pan. Spread half the batter in the pan, and lay half the topping mixture over it with your fingers in chunks of 1/2"-3/4" (don't stress over the size--just go for chunks). Repeat with remaining batter and topping.

Bake at 350 degrees for 60 minutes, covering it halfway through with foil to prevent the topping from over-browning. Test with a toothpick. If it doesn't come out clean, bake another 10 minutes and test again. It may need another 10 minutes; oven temperatures do vary.