Heat oven to 350 degree F. Coat 9 x 9 x 2-inch-square baking pan with cooking spray. In bowl, whisk together flour, baking powder, soda and salt.
In large bowl, beat butter until smooth and creamy. Beat in sugar until light and fluffy, about 2 minutes. Beat in egg whites, 1 at a time, then vanilla.
On low speed, working in batches, beat flour mixture alternately with yogurt into butter mixture until smooth. Fold in toffee pieces. Scrape into prepared pan, smoothing top.
Bake in 350 degree F oven until toothpick comes out clean, about 25 minutes. Let cake cool in pan on rack 10 minutes. Invert cake; remove pan; invert cake again. Let cool.
Cover with whipped topping and sprinkle with cinnamon, if desired. Serve or refrigerate until serving. Makes 12 servings.
