Heat oven to 350°F. Spray 9-inch round cake pan with nonstick cooking spray and lightly flour. In small bowl, dissolve coffee granules in water. Set aside.
In large bowl, beat 2/3 cup sugar and margarine until well blended. Add yogurt, egg whites and coffee mixture; mix well. Lightly spoon flour into measuring cup; level off. Add flour and all remaining cake ingredients; mix just until blended. Pour into sprayed and floured pan.
Bake at 350°F. for 22 to 32 minutes or until toothpick inserted in center comes out clean. Cool 10 minutes. Invert cake onto plate; remove pan. If desired, sprinkle with powdered sugar.
Meanwhile, in small saucepan over very low heat, melt chocolate chips, stirring constantly. Stir in 1/3 cup sugar and dash of salt. Gradually add half-and-half, stirring constantly. Cook until thickened and hot, stirring constantly. Cool slightly. Serve warm sauce over cake wedges.