Combine blueberries and 1/4 cup sugar in a bowl; gently mash with a potato masher until blueberries are crushed. Let stand 5 minutes. Set aside.
Combine wafer crumbs, brown sugar, margarine, and hot water; stir until well-blended. Set aside.
Combine flour, 1/3 cup sugar, baking powder, baking soda, and salt in a large bowl. Combine yogurt, oil, vanilla, egg, and egg white; stir well with a whisk. Add to flour mixture, stirring until well-blended. Fold in blueberry mixture. Pour batter into a 9-inch round cake pan coated with cooking spray. Bake at 350° for 30 minutes. Sprinkle brown sugar mixture over cake; bake an additional 10 minutes or until a wooden pick inserted in center comes out clean. Let cool on a wire rack.
While this was one of the moistest cakes I've ever had, it was surprisingly not very sweet and I was disappointed with the streusel. This didn't make very much streusel topping. I think I'll add a little more to it next time. This wasn't bad, but could be changed a little more to make this irresistible.
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