Combine brown sugar, cinnamon, and chopped nuts in a small bowl to create the streusel topping.

Set the streusel topping aside.

Sift together flour, sugar, baking powder, baking soda, and salt in a large mixing bowl.

Add softened butter, sour cream, and vanilla extract to the dry ingredients.

Mix with an electric mixer on low speed until the ingredients are just combined (about 30 seconds).

Scrape down the sides of the bowl to ensure even mixing.

Beat the batter at medium speed for 1 1/2 minutes until smooth and well combined.

Scrape down the bowl again.

Add the eggs, one at a time, beating on low speed for about 15 seconds after each addition to incorporate each egg fully.

Increase the mixer speed to medium and beat for another 30 seconds.

Grease and flour a 9x13 inch oblong pan or a 10-inch tube pan to prevent sticking.

Spread half of the cake batter evenly into the prepared pan.

Sprinkle half of the cinnamon-sugar-nut mixture (streusel topping) over the batter.

Spread the remaining cake batter over the streusel layer, covering it evenly.

Top with the remaining cinnamon-sugar-nut mixture.

Bake in a preheated oven at 350 degrees Fahrenheit (175 degrees Celsius) for 50 to 60 minutes, or until a toothpick inserted into the center comes out clean.

Let the coffee cake cool in the pan for 10 minutes before slicing and serving.