PREHEAT oven to 350 degrees F. Butter a 9-inch square baking pan.
SIFT together in a large bowl flour and baking powder. In another large bowl with an electric mixer beat butter and margarine until light, about 2 minutes. Gradually beat in 2 cups sugar until combined well, about 3 minutes. Add eggs, 1 at a time, beating well after each addition. Beat in almond extract and vanilla. Add flour mixture to butter mixture in batches alternately with sour cream, beginning and ending with flour mixture and beating until batter is blended well.
SPREAD batter into pan, smoothing top. In a small bowl combine remaining 2 1/2 teaspoons sugar and cinnamon and sprinkle over batter. Bake cake in middle of oven until a tester comes out clean, about 55 minutes.