Preheat oven to 350°. Coat a 13x9-inch baking dish with cooking spray. Melt chocolate and butter in a heatproof bowl set over a saucepan of barely simmering water, stirring occasionally; do not let bottom of bowl touch water. Whisk flours, cocoa, salt and baking soda in a medium bowl. In a large bowl, whisk eggs and brown sugar until smooth, then add yogurt, applesauce, oil, vanilla and chocolate chips; whisk to combine. Whisk in melted chocolate mixture until blended. Add dry ingredients and mix until just moistened. Spread batter in prepared pan and sprinkle with walnuts, if desired. Bake for 30 to 35 minutes, or until a toothpick inserted in center comes out with a few moist crumbs. Cool completely in pan on a rack before slicing.
