Unroll pie crust and fit into a deep 9-inch pie pan. Prebake crust according to package directions. Remove from oven to cool.
Lower the oven temperature to 325℉.
In a medium bowl, beat together the butter, sugar, and salt. Add cocoa powder and beat until blended. Add eggs and vanilla and blend until mixed well.
Scrape into cooled pie crust. Bake for 25 to 30 minutes or until set.
Allow to cool to room temperature. Serve with sweetened whipped cream, if desired.