Sift together the flour, baking powder and baking soda. In a mixing bowl cream the margarine and add the sugar, blending well. Beat in eggs and vanilla. Add sifted dry ingredients alternately with the sour cream, beginning and ending with the dry ingredients. Combine lightly after each addition. Don't beat.
Grease an eight-inch cake pan and dust lightly with flour. Spoon half the batter into the pan and spread evenly.
Sprinkle with half of a mixture of 1/4 cup lightly-packed brown sugar, one teaspoon cinnamon and two tablespoons finely chopped nuts.
Cover with remaining batter. Sprinkle with remaining topping.
Bake in preheated 350 degree oven for 45 to 50 minutes.
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