1. Microwave butter in small microwave-safe bowl at HIGH (100%) 1 minute or until melted. Add brown sugar, water and cornstarch; stir with whisk until smooth. Microwave at HIGH 1 minute or until mixture comes to full boil.
2. Stir in chopped pecans; spread mixture on bottom of crust. Place crust in freezer while preparing filling. Prepare CHOCOLATE MOUSSE FILLING. Carefully spread filling over pecan layer.
3. Cover; refrigerate 3 to 4 hours. Garnish with pecan halves. Cover; refrigerate leftover pie. 6 to 8 servings.
CHOCOLATE MOUSSE FILLING
1 teaspoon unflavored gelatin
1 tablespoon cold water
2 tablespoons boiling water
1/2 cup sugar
1/4 cup HERSHEY'S Cocoa
1 cup (1/2 pt.) cold whipping cream
1 teaspoon vanilla extract
Sprinkle gelatin over cold water in small cup; let stand 1 minute to soften. Add boiling water; stir until gelatin is completely dissolved and mixture is clear. Cool slightly, about 5 minutes. Meanwhile, stir together sugar and cocoa in medium bowl; add whipping cream and vanilla. Beat on medium speed of mixer, scraping bottom of bowl occasionally, until stiff. Add gelatin mixture; beat just until well blended.