Mix together sugar, flour, and dry cocoa.

Melt butter in 10" skillet. Add sugar, flour, cocoa mixture; mix lightly.

In separate bowl, combine beaten egg yolks with milk. Add to butter mixture, stirring constantly.

Cook slowing until really thick (takes a long time).

Remove from heat; add vanilla, blend well, and put into baked pie shell.

Chill in refrigerator until firm. Serve topped with whipped cream.