1. Preheat oven to 350 degrees
2. In a medium saucepan, add sugar, cocoa and flour and mix until blended.
3. Stir in evaporated milk, vanilla and beaten egg yolks.
4. Add butter and warm, stirring constantly until butter is melted.
5. Pour mixture into prepared unbaked pie shell.
6. Bake for about 30 minutes, or until pie filling is firm.
7. To make meringue, in a medium bowl beat egg whites and gradually add 1/4 cup sugar until stiff peaks form.
8. Top pie with meringue and return to oven until merinque is golden brown.
9. Cool throughly before serving.