1. Sprinkle gelatin over 1 cup milk in medium saucepan; stand several minutes to soften.
2. Stir together sugar and cocoa; add to milk mixture. Add corn syrup. Cook, stirring constantly, until mixture boils. Remove from heat. Add butter; stir until melted. Stir in remaining 3/4 cup milk and vanilla. Pour into large bowl. Cool; refrigerate until almost set.
3. Beat whipping cream in small bowl on high speed of mixer until stiff. Beat chocolate mixture on medium speed until smooth. On low speed, add whipped cream to chocolate mixture, beating just until blended. Pour into prepared crust; refrigerate* until set, at least 3 hours. Just before serving, drizzle each pie slice with chocolate syrup. Cover; refrigerate leftover pie. 8 servings.
* Variation: FROZEN CHOCOLATE BAVARIAN PIE: Freeze pie 4 to 6 hours or overnight. Remove from freezer 10 to 15 minutes before serving.