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1
Place a rack in the upper third of the oven and preheat to 350u00b0F. Grease and flour a 9- by 13-inch baking pan and set aside.
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2
To make the topping: whisk together sugar, salt, flour, and cinnamon. Add the butter and stir until thoroughly combined and the mixture resembles moist corn meal. Set aside.
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3
To make the filling: whisk together the brown sugar, cinnamon, nutmeg, cocoa powder, and salt. Set aside.
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4
To make the cake: in the bowl of a stand mixer with a paddle attachment, beat the butter and sugars together until well combined, about 4 minutes. Beat in vanilla.
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5
Add the eggs, one at a time, beating for 1 minute after each addition. Scrape the bowl down between each egg addition to ensure that everything is properly mixed. In a small bowl, whisk together flour, baking powder, and salt. Set aside.
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6
In another small bowl, whisk together the sour cream and milk. It may be a bit lumpy. That's okay. On low speed, add 1/3 of the flour mixture to the batter. As it incorporates, add half of the sour cream mixture. Add another 1/3 of the flour mixture and the remainder of the sour cream mixture. Mix until barely incorporated and add the remaining flour. Finish incorporating the dry ingredients with a spatula to ensure that all of the flour is well incorporated.
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7
Spread about half of the batter into the prepared pan, pushing it to the edges with a rubber spatula. Sprinkle the filling on top of the batter.
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8
Dollop the remaining batter across the top of the filling and use a spatula to spread batter across pan. Use a butter knife to gently swirl the filling into the batter with a few gentle strokes.
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9
Sprinkle the topping evenly over the batter and bake the cake until is golden brown and a cake tester or wooden skewer inserted into center comes out clean, 55 to 60 minutes.
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10
Remove from the oven and allow to rest for 20 to 30 minutes before serving. Cake will last, well wrapped, at room temperature for 4 to 5 days.
