Preheat oven to 350. Prepare a 8" square baking pan with cooking spray and flour. To prepare chocolate peanut butter swirl, melt the chocolate chips and peanut butter in a small saucepan over low heat, stirring often until smooth. Remove from the heat; let cool while preparing the crumbs and batter. To prepare crumbs, combine the flour, brown sugar, margarine, and peanut butter in a mixing bowl until the mixture forms crumbs. Remove 3/4 cup (tightly packed) of the mixture and set aside. To prepare cake, add egg whites, milk, vanilla, baking powder, and baking soda to the remaining crumbs in the mixing bowl until thoroughly moistened. Spread half of the batter in the prepared pan; gently streak half of the chocolate peanut butter mixture over the top. Repeat with the remaining batter and chocolate peanut butter mixture. Scatter the reserved crumbs over the top, pressing them in gently so they adhere to the batter. Bake for about 50 minutes or until crumbs and top of the cake are firm. Cover the pan loosely with aluminum foil, if the cake is getting too dark during baking, loosely cover it with a piece of aluminum foil. Let the cake cool in the pan on a wire rack for 15 minutes. Loosen around the sides of the pan, then carefully invert onto the rack and turn right-side-up. Cool completely before serving. ----