1. Preheat oven to 350°. Roll piecrust into a 13-inch circle on a lightly floured surface. Fit into a 9-inch pie plate; fold edges under, and crimp.
2. Microwave butter and chocolate squares in a large microwave-safe bowl at MEDIUM (50% power) 1 1/2 minutes or until melted and smooth, stirring at 30-second intervals. Whisk in evaporated milk, eggs, and 1 tsp. vanilla.
3. Stir together granulated sugar and next 3 ingredients. Add sugar mixture to chocolate mixture, whisking until smooth. Pour mixture into prepared crust.
4. Bake pie at 350° for 40 minutes. Stir together pecans, next 2 ingredients, and remaining 1 tsp. vanilla; sprinkle over pie. Bake 10 more minutes or until set. Remove from oven to a wire rack, and cool completely (about 1 hour).