This is an absolutely delicious recipe of my grandmother's she used to make when there would be lots of people over - and believe me, there needed to be lots of people there. This pie is rich and thick and probably has about ten thousand calories a slice, but it's totally worth it.
2 cups sugar
4 tablespoons cocoa powder (get Hershey's or another decent brand. You can taste the difference)
2 sticks butter
1/4 cup flour
1 teaspoon vanilla extract
Beat eggs until frothy. This means frothy, not scrambled. As a rule, just keep beating the things until your arms get too tired to go on.
Gradually beat in sugar. The sugar will slowly be incorporated into the eggs, though it'll still be grainy. I haven't tried using powdered sugar instead - it might make a difference, but in any case the graininess won't matter in the end.
Melt cocoa powder into the butter, stir. Add to sugar and eggs, beating continually.
Stir in flour, vanilla. Beat until well-mixed.
Pour into ungreased 9"x9" pan or pre-prepared pie crust. Place pan or pie tin inside a shallow pan of hot water - this keeps the pie from burning as it cooks.
Bake 45 minutes at 375 degrees. The pie will have a thin, crisp, light brown layer on top of a thick, rich dark brown one. Let it cool for several hours before serving - for best results serve warm.