Preheat oven to 350 degrees F (175 degrees C). Grease a 9-inch round cake pan.
In a medium bowl, whisk together flour, baking powder, baking soda, and salt.
In a large bowl, cream together butter and sugar until light and fluffy. Beat in eggs one at a time until well incorporated. Add in vanilla extract and mix until combined.
Gradually beat in dry ingredients until just combined. Stir in sour cream until no lumps remain.
Pour batter into prepared pan. In a separate bowl, combine walnuts, brown sugar, cinnamon, and nutmeg. Sprinkle over batter in pan.
Bake for 35 to 40 minutes, or until a toothpick inserted into the center of the cake comes out clean. Allow cake to cool in pan before serving
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