In a heavy saucepan, combine flour, sugar, salt and cocoa. Mix well then move to cook top. On medium heat, gradually stir in water and milk. Stir until smooth - 1 to 2 minutes. Remove from heat. Whisk egg yolks in separate bowl. Slowly add yolks and bring to a boil, whisking continually until thickened.
Remove from heat, stir in butter and vanilla. Allow to cool for about 5 minutes. Pour into custard cups or parfait glasses. Refrigerate until ready to use.
Top with whipped cream. Garnish with a cookie.
*This also makes a fabulous pie filling. Prepare and bake pie pastry. Allow to cool. Then pour custard in and refrigerate. Top with whipped cream. Serve cold.