In a small bowl, pour boiling water over rolled oats; let set 20 minutes, or until water is completely absorbed. Preheat oven to 325. Prepare a 9" square pan with cooking spray; set aside. In a mixing bowl, combine oat mixture, margarine, honey, vanilla extract, eggs, and yogurt. Mix well; set aside. In a small bowl, combine flour, baking soda, cinnamon, and salt. Add to oat mixture. Mix well. Pour batter into prepared pan. Bake for 55 minutes, or until toothpick inserted in center comes out clean. After cake has completely cooled, spread with frosting.