FOR CHOCOLATE PIE FILLING:
Preheat oven to 350 degrees.
Mix all dry ingredients (cocoa, flour, and sugar) together in a boiler. Stir to blend well.
Add slightly beaten egg yolks into the dry ingredients. Slowly add the milk a bit at a time until well blended, Over LOW heat, cook stirring CONSTANTLY until thickened (about 5 minutes.) Remove from heat.
Add vanilla and butter and stir to blend well. Pour into a baked 9 inch pie crust.
TO PREPARE THE MERINGUE:
Put the 3 egg whites in a small deep bowl. (I use a chilled aluminum bowl.) Beat on high until medium soft peaks begin to form.
Slowly add the sugar and beat until the egg whites are glossy and holds firm, stiff peaks.
Spoon over the chocolate mixture in the pie crust, pulling the meringue into peaks. Bake in a preheated oven until lightly browned.
Allow to cool, then refrigerate until ready to serve.