Lightly grease an 8x8 inch baking pan. Preheat oven to 350 degrees F.

Crumble topping: Combine flour, brown sugar and cinnamon in a bowl. Add butter and cut in with forks or a pastry blender until there are no large pieces. Stir in nuts.

Batter: In a mixing bowl, cream together butter and sugar until pale and fluffy. Add egg and mix. Add sour cream, milk, and vanilla then mix. In a separate bowl, whisk together the flour, salt, baking soda, and baking powder. Add dry ingredients to wet mixture then mix just until combined.

Assemble: Spoon half of the batter into your prepared pan and smooth into an even bottom layer. Sprinkle half of the topping mixture over the top. Spoon remaining batter on top then smooth into an even layer. Sprinkle remaining topping mixture over the top.

Bake in preheated oven for 40-50 minutes or until a toothpick inserted into the center comes out clean, or with few crumbs. Check it half way through baking and cover with a piece of aluminum foil if browning too much.

Cool for at least 10 minutes before serving.