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1
Make the topping: Line a baking sheet with parchment paper.
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2
In a medium bowl, stir together the walnuts, pecans, flour, granulated and brown sugars, cinnamon, and salt.
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3
Stir in the butter.
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4
The topping should form sandy clumps.
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5
Sprinkle the topping onto the baking sheet to break it up into smaller clumps.
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6
Refrigerate.
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7
Preheat the oven to 350 F. Use butter to thoroughly grease the bottom and sides of a 9-inch round baking pan.
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8
Make the cake batter: In the bowl of a stand mixer fitted with the whisk attachment, beat the butter and sugar together, on medium-high, scraping down the sides from time to time, for 8 minutes.
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9
Lower the speed and add the eggs, one by one, taking care that the first egg is thoroughly integrated before adding the second.
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10
Sift together the flour, baking powder, and baking soda.
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11
In a separate bowl, whisk together the salt, sour cream, and yogurt until smooth.
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12
Turn the mixer on low and alternate adding some of the flour mixture with some of the sour cream mixture.
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13
When all has been mixed in, give the sides and the bottom of the bowl a good scrape and blend to make sure the batter is thoroughly combined.
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14
Transfer the batter to the greased baking pan and tap it lightly on the sides so it falls evenly in the pan and to remove any air bubbles.
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15
Liberally sprinkle the cake with all of the topping.
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16
Bake the cake: Bake until a cake tester or small knife inserted in the center comes out clean, 55 to 60 minutes.
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17
Allow the cake to cool for at least 15 minutes before slicing and serving.
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18
Once cool, the cake will keep, covered, for up to 3 days.
