1. Preheat oven to 350°. Lightly coat a 9-inch pieplate with cooking spray, and dust with cocoa.
2. Melt chopped chocolate and margarine in a heavy saucepan over low heat, stirring constantly, 3 to 5 minutes or until smooth; whisk in 3/4 cup sugar, whisking until blended.
3. Whisk together eggs and vanilla in a large bowl. Gradually whisk in chocolate mixture, whisking until blended. Pour into prepared pieplate.
4. Bake at 350° for 30 minutes or until edges are dry and center is set. (Do not overbake.) Cool in pieplate on a wire rack 30 minutes or until completely cool. (Cake will fall slightly when removed from oven.) Dust with 2 Tbsp. powdered sugar just before serving.