ingredients

cake

• 12 tablespoons (170g) butter, at room temperature, at least 65°F
• 1 teaspoon salt, (1 1/4 teaspoons if you use unsalted butter)
• 1 1/2 cups (298g) granulated sugar
• 1/3 cup (71g) light brown sugar or dark brown sugar, packed
• 2 1/2 teaspoons baking powder
• 2 teaspoons vanilla extract
• 3 large eggs, at room temperature
• 3/4 cup (170g) sour cream or plain yogurt, at room temperature
• 1 1/4 cups (283g) milk, anything from skim to whole, at room temperature
• 3 3/4 cups (450g) unbleached all purpose flour
  

filling

• 1 cup (213g) dark brown sugar or light brown sugar, packed
• 1 1/2 tablespoons cinnamon
• 1 teaspoon unsweetened cocoa
  

topping

• 1 cup (198g) granulated sugar
• 1/4 teaspoon salt, (if you use unsalted butter)
• 1 cup (120g) unbleached all-purpose flour
• 1 tablespoon cinnamon
• 6 tablespoons (85g) butter, melted

instructions

1. Preheat the oven to 350°F. Lightly grease a 9″ x 13″ pan, or two 9″ round cake pans.
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2. 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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3. 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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4. 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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5. Add the eggs one at a time, beating well after each addition.
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6. 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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7. Add the flour to the butter mixture alternately with the milk/sour cream mixture, beating gently to combine.
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8. 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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9. Sprinkle the filling evenly on the batter.
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10. 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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11. Sprinkle the topping over the batter in the pan.
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12. 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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13. Remove the cake from the oven and allow it to cool for 20 minutes before cutting and serving. Serve cake right from the pan.

Tips from our [King Arthur’s] Bakers lol

• Dress it up for guests: Make a glaze with ½ cup confectioners’ or glazing sugar and 1 tablespoon milk; drizzle glaze over the cooled coffeecake.
  
• It’s easy to spread half the batter in the pan when you know how much it weighs. If you have a kitchen scale, half the batter weighs about 28 ounces.
  
• Want to prepare this coffeecake the night before, then bake in the morning? It’s easy; simply cover the unbaked cake with plastic wrap, and refrigerate overnight. Next morning, bake the cake as directed. Start testing for doneness at the end of the suggested baking time; you’ll probably have to add 5 minutes or so to the total time, to account for the batter being chilled.

From: https://www.kingarthurbaking.com/recipes/cinnamon-streusel-coffeecake-recipe