Apple Crumb Coffee Cake

This dish has quickly risen to prominence in our home since its discovery two months ago, inspired by some marked-down apples that had to be used up immediately. It's quick to whip up, keeps for several days, uses fairly basic and cheap ingredients (at least in my stripped-down version), is pretty nutritious, and makes a simple, easy, yet filling breakfast food.

Apple Crumb Coffee Cake

1 Tbl. dry bread crumbs
2 cups flour (I used half whole wheat and half white until I realised that the cinnamon and apples are going to turn it brown anyway)
1 cup plus 2 Tbl. sugar (I just use one cup)
1 tsp. salt
10 Tbl. (1 1/4 sticks) butter, softened (I use half of this, or maybe 1 stick, depending on how much I have out on the butter dish. I use less butter not only because it's healthier but also cheaper, since I always use real butter in baking. Maybe the texture or taste would be better with more butter, but since I've never tried it, I don't know what I'm missing!)
1 tsp. baking powder
1/2 tsp. baking soda
1 teaspoon ground cinnamon
3/4 cup buttermilk or low-fat (not nonfat) plain yogurt, room temperature (I make my own buttermilk using the following formula: 1 Tbl. vinegar plus enough milk to make one cup = one cup of buttermilk)
1 large egg, room temperature (but I usually use a medium egg because that's what I have on hand)
1 teaspoon vanilla extract
2 medium-large Granny Smith apples, peeled, cored, and chopped (I've used one sometimes)
3/4 cup walnuts or pecans, chopped (I never use this much)
1/2 cup dark brown sugar

-Preheat oven to 350. Butter a 9x13 pan and shake breadcrumbs over to coat. Tap out extra crumbs.
-Whisk flour, sugar, and salt in large mixing bowl. Add butter and cut in with pastry knife until mixture resembles coarse crumbs; remove 1 cup to separate bowl and reserve.
-Whisk baking powder, soda, and cinnamon into flour mixture in bowl. Add buttermilk or yogurt, egg, and vanilla; whisk until smooth and frostinglike, 1 1/2 to 2 minutes.
-Stir in apples.
-Pour into pan and smooth top.
-Add nuts and brown sugar to reserved cup of flour mixture. Stir until blended and spread over cake batter, pressing lightly so it adheres.
-Bake cake until center is firm and cake tester comes out clean, 50 to 55 minutes.
-Enjoy!