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