Preheat your oven to 350°F (175°C) and grease a 9-inch round cake pan.
In a large bowl, cream the softened butter and granulated sugar until light and fluffy.
Mix in the eggs one at a time, then add the vanilla extract.
In another bowl, combine the flour, baking powder, baking soda, salt, and cinnamon.
Gradually add the dry ingredients to the wet mixture, alternating with sour cream.
Pour half of the batter into the prepared cake pan.
In a separate bowl, mix brown sugar, cinnamon, and nuts for the topping.
Sprinkle half of the topping mixture over the batter in the pan.
Pour the remaining batter over the topping and finish with the rest of the streusel.
Bake for 35-40 minutes, or until a toothpick inserted in the center comes out clean.