For the filling: melt the butter and chocolate in the microwave; stir until smooth,then stir in vanilla. Set aside.
In a small saucepan, combine the sugar, cornstarch, and milk until smooth. Cook and stir over medium-high heat until thickened and bubbly. Reduce heat, then cook and stir for 2 minutes longer; remove from heat.
Stir a small amount of hot liquid into the egg yolks and blend together. Return to the pan, stirring constantly (this step is to ensure the eggs don't "scramble" when they come in contact with the hot liquid).
Bring to a gentle boil, then cook and stir 2 minutes longer.
Remove from heat and gently fold in the chocolate mixture. Spoon into baked pie shell.
For the topping: In a small saucepan, combine evaporated mil, sugar and butter. Cook and stir until butter is melted and mixture comes to a boil.
Remove from heat. Again, stir a small amount of hot mixture into the egg and return to pan, stirring constantly.
Bring to a boil and cook for 2 minutes, stirring constantly.
Remove from heat. Stir in pecans and coconut. Pour over the filling. Cool, cover and chill before eating.