Special equipment: One 13 by 9-inch baking pan
For the topping: Line a baking sheet with parchment paper. In a medium bowl, combine the nuts, flour, sugars, cinnamon and salt. Stir to blend. Stir in the butter. The topping should form sandy clumps. Sprinkle the topping onto the baking sheet to break it up into smaller clumps. Refrigerate.
For the coffee cake: Preheat the oven to 350 degrees F. Use butter to thoroughly grease the bottom and sides of a 13- by 9-inch baking pan. Set aside.
In the bowl of an electric mixer fitted with the whisk attachment, cream the butter and granulated sugar together until light yellow and smooth, 5 to 8 minutes. Alternatively, this can be done with an electric hand mixer. Add the eggs, one at a time, and beat until thoroughly integrated.
Remove the topping from the refrigerator so it has a few minutes to come to room temperature before baking. Meanwhile, sift together the flour, baking powder, baking soda and salt. In a separate bowl, whisk together the buttermilk and yogurt until smooth. Turn the mixer on low and alternate, adding some of the flour mixture with some of the sour cream mixture. When all has been mixed, give the sides and the bottom of the bowl a scrape and blend to make sure the batter is thoroughly integrated. Transfer the batter to the greased baking pan, spreading it out evenly. Liberally sprinkle the cake with all of the topping. Take care to cover the whole top. Put the pan in the center of the oven and bake until a toothpick inserted in the center comes out clean, 40 to 50 minutes. Remove the cake from the oven and allow to cool for 30 minutes before serving.
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