Heat oven to 375°F. Spray 9 or 10-inch springform pan with nonstick cooking spray. In medium microwave-safe bowl, microwave whipping cream on HIGH for 45 to 60 seconds or until warm. Add white chocolate and 1/4 to 1/2 teaspoon mint extract; stir until chocolate is melted. Refrigerate at least 1 hour or until well chilled.
Meanwhile, in large bowl, combine brownie mix, water, oil, 1/2 to 1 teaspoon mint extract and egg yolks; beat 50 strokes with spoon. In small bowl, beat egg whites until soft peaks form. Gradually fold into brownie mixture. Pour batter into sprayed pan.
Bake at 375°F. for 32 to 38 minutes or until center is almost set. Cool 30 minutes. (Center will sink slightly.) Carefully remove sides of pan. Sprinkle top of cake with powdered sugar.
Just before serving, beat chilled mint cream until soft peaks form. Cut cake into wedges; top each wedge with mint cream. Garnish with mint sprigs. Refrigerate leftover mint cream.