Melt margarine in a small saucepan over medium heat; stir in cocoa. Pour mixture into a large bowl. Add sugar and next 4 ingredients; stir well.
Spread batter in a 9-inch round pan coated with cooking spray. Bake at 350° for 20 minutes or until a wooden pick inserted in center comes out clean. Let cool in pan on a wire rack. Cut into 8 wedges.
Drizzle pre-melted chocolate evenly onto 8 dessert plates. Place a brownie wedge on each plate, and top each wedge with 1/2 cup frozen yogurt. Garnish each with a cookie half, if desired.