This wonderfully easy cake calls for less butter and fewer eggs than most versions.
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.