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1
For the topping: Line a baking sheet with parchment paper.
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2
In a medium bowl, combine the nuts, flour, sugars, cinnamon and salt.
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3
Stir to blend.
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4
Stir in the butter.
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5
The topping should form sandy clumps.
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6
Sprinkle the topping onto the baking sheet to break it up into smaller clumps.
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7
Refrigerate.
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8
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.
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9
Set aside.
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10
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.
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11
Alternatively, this can be done with an electric hand mixer.
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12
Add the eggs, one at a time, and beat until thoroughly integrated.
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13
Remove the topping from the refrigerator so it has a few minutes to come to room temperature before baking.
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14
Meanwhile, sift together the flour, baking powder, baking soda and salt.
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15
In a separate bowl, whisk together the buttermilk and yogurt until smooth.
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16
Turn the mixer on low and alternate, adding some of the flour mixture with some of the sour cream mixture.
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17
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.
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18
Transfer the batter to the greased baking pan, spreading it out evenly.
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19
Liberally sprinkle the cake with all of the topping.
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20
Take care to cover the whole top.
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21
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.
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22
Remove the cake from the oven and allow to cool for 30 minutes before serving.
