Preheat oven to 350 degrees F. Butter and flour an 8x8-inch cake pan. In a small bowl, stir the baking soda into the sour cream and let stand for 5 to 10 minutes. (The sour cream will increase slightly in volume.) While the mixture is resting, stir together the cinnamon-nut mixture and set aside. In a large mixing bowl, cream the butter for 2 minutes with an electric mixer, then gradually add the sugar. Add the egg and beat until thoroughly combined. Add the sour-cream mixture, then beat in the flour and baking powder. Beat at medium speed for 2 minutes.
Spoon half the batter into the prepared pan. Sprinkle with the cinnamon-nut mixture. Smooth the remaining batter into the pan. Bake for 30 to 40 minutes, or until a toothpick inserted into the center comes out clean. Let stand for 10 minutes before removing from pan. Serve lukewarm or cold.