Old Fashioned Chocolate Pie

Stove top instructions
0 Combine sugar and milk in a large bowl; whisk until smooth.
1 Mix the flour, salt, and cocoa in a small mixing bowl and then carefully whisk into the milk mixture.
2 Cook over medium low heat stirring constantly until mixture thickens.
3 Whisk some of the hot mixture to the egg yolks and then quickly stir the egg yolks into the pudding mixture.
4 Heat for approximately one more minute.  Mixture will be quite thick and will coat the back of the spoon.
5 Remove from heat and add the butter.
6 Stir until melted and add the vanilla.
Microwave instructions
13 Mix all of the ingredients together except the egg yolks, butter, and vanilla.
14 Microwave on 100%  for 3 minutes.
15 Stir. 
16 Microwave on 50% until the mixture thickens. 
17 Whisk some of the hot mixture into the egg yolks then whisk the egg yolks back into the pudding.
18 Microwave on 50% for 2 minutes, or until the mixture is thick and creamy.
19 Stir in the vanilla and butter.
Assembling the pie
7 Pour the hot filling into baked pie shell.
9 Spray plastic wrap with no stick cooking spray and put it carefully, spray side down, over the surface of the filling while it's hot.
8 Chill several hours or overnight.