Whisk together the sugar, cornstarch, dutch process cocoa powder, and salt in a 2 to 3 quart, heavy-bottomed saucepan until evenly coloured. In a separate measuring cup or bowl, whisk together the egg yolks and whole milk until evenly yellow. Slowly whisk that into the sugar mixture, drizzling in the milk mixture while whisking constantly. Place over medium heat and whisk constantly until it is thickened and bubbly, about 6 to 8 minutes. Immediately drop the heat to low and let it bubble for 1 more minute. Remove from the heat and whisk in the chocolate until it is fully melted. Pour through a fine mesh sieve into a heat proof bowl. Whisk in the Kahlua and vanilla until smooth. Pour the pudding into the fully cooked pie crust, lay plastic wrap directly on the surface, and cool for at least 5 hours, or until it is thoroughly chilled and set.