Simple Cinnamon Streusel Coffee Cake

from America’s Test Kitchen Family Cookbook

I really wanted a coffee cake yesterday, so I looked for a good one to make. This is the one I made. I didn't have the pan size they said, so I had to adjust the cooking time, which is reflected below. I didn't have buttermilk, so I added a tablespoon of white vinegar to milk to sour it. They suggest using plain yogurt, which I would bet is awesome! This is a great coffee cake! 

Topping

⅓ cup packed light brown sugar

⅓ cup granulated sugar

1 tbsp cinnamon

1 tbsp unsalted butter, melted and cooled

1 cup pecans, almonds, or walnuts, chopped coarse

Cake

3 cups all-purpose flour

1 tbsp baking powder

1 tsp baking soda

1 tsp cinnamon

¼ tsp salt

1¾ cups buttermilk or plain yogurt

1 cup packed light brown sugar

1 cup granulated sugar

3 large eggs

7 tbsp unsalted butter, melted and cooled

1.  For the topping: Mix the brown sugar, granulated sugar, cinnamon, and melted butter together in a medium bowl until the mixture resembles wet sand. Stir in the nuts.
2. Preheat the oven to 350°.
3. Generously grease a 9 by 9-inch baking pan.
4. For the cake: Mix the flour, baking powder, baking soda, cinnamon, and salt together in a large bowl.
5. Whisk the buttermilk, brown sugar, granulated sugar, eggs, and melted butter together in a separate bowl until smooth.
6. Gently fold the egg mixture into the flour mixture with a rubber spatula and stir until the batter looks smooth and well combined.
7. Scrape the batter into the prepared pan and smooth the top.
8. Sprinkle the topping evenly over the batter.
9. Bake until the top is golden and a toothpick inserted into the center comes out with just a few crumbs attached, 45-50 minutes.
10. Let the cake cool on a wire rack for 15 minutes before serving.