Preheat the oven to 350°F (175°C).

Grease a 9x13 inch baking pan with butter or non-stick spray.

In a large mixing bowl, whisk together flour, baking powder, baking soda, and salt.

In another bowl, beat the softened butter and granulated sugar until light and fluffy.

Add eggs one at a time, mixing well after each, then add the vanilla extract.

Gradually add the dry ingredients to the wet mixture, alternating with the sour cream.

In a small bowl, mix the brown sugar, cinnamon, and chopped walnuts for the topping.

Pour half of the batter into the prepared pan, sprinkle half of the topping, then add the remaining batter and the rest of the topping.

Bake for 30-35 minutes or until a toothpick comes out clean.

Let the coffee cake cool in the pan for a few minutes before transferring to a cooling rack.