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Preheat the oven to 350°F. Lightly grease a 9" x 13" pan, or two 9" round cake pans.
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Make the topping by whisking together the sugar, salt, flour, and cinnamon. Add the melted butter, stirring until well combined. Set the topping aside.
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Make the filling by mixing together the brown sugar, cinnamon, and cocoa powder. Note that the cocoa powder is used strictly for color, not flavor; leave it out if you like. Set it aside.
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To make the cake, in a large bowl, beat together the butter, salt, sugars, baking powder, and vanilla until well combined and smooth.
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Add the eggs one at a time, beating well after each addition.
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In a separate bowl, whisk together the sour cream or yogurt and milk till well combined. You don't need to whisk out all the lumps.
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Add the flour to the butter mixture alternately with the milk/sour cream mixture, beating gently to combine.
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Pour/spread half the batter (a scant 3 cups) into the prepared pan(s), spreading all the way to the edges. If you're using two 9" round pans, spread 1 1/3 cups batter in each pan.
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Sprinkle the filling evenly on the batter.
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Spread the remaining batter atop the filling. Use a table knife to gently swirl the filling into the batter, as though you were making a marble cake. Don't combine filling and batter thoroughly; just swirl the filling through the batter.
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Sprinkle the topping over the batter in the pan.
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Bake the cake until it's a dark golden brown around the edges; medium-golden with no light patches showing on top, and a toothpick or cake tester inserted into the center comes out clean, about 55 to 60 minutes for the 9" x 13" pan, 50 to 55 minutes for the 9" round pans. When pressed gently in the middle, the cake should spring back.
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Remove the cake from the oven and allow it to cool for 20 minutes before cutting and serving. Serve cake right from the pan.
