Preheat oven to 350° (F). Roll pie crust into a 13-inch circle on a lightly floured surface. Fit into 9-inch pie plate. Trim pastry to ½ inch of the outside rim. Fold edges under, and crimp.
Microwave butter and unsweetened chocolate squares in a large microwave-safe bowl at MEDIUM (50% power) 1½ minutes, or until melted and smooth, stirring at 30-second intervals. Whisk in evaporated milk. Then whisk in eggs and 1 teaspoon of vanilla.
In a medium bowl, stir together granulated sugar, unsweetened cocoa powder, all-purpose flour, and salt. Add liquid chocolate mixture to dry ingredients. Whisk until smooth.
Pour into prepared crust.
Bake pie in preheated 350° (F) for 40 minutes. While pie is baking, in a medium bowl, combine pecans, light brown sugar, light corn syrup and remaining 1 teaspoon of vanilla.
Carefully slide pie out of oven and sprinkle top of pie with sugar-coated pecans. Bake 10 more minutes or until set. Remove from oven wire rack, and cool completely (about 1 hour).
To prepare homemade whipped cream, add heavy cream to bowl of stand mixer. Add vanilla. Use the wire whisk attachment and mix on high for 1–2 minutes. Add powder sugar. Scrape downsides of bowl and continue mixing on high until stiff peaks form. Store in airtight container until ready to serve.