Chocolate pie

Ingredients:

• 3 tablespoons unsweetened cocoa
• 1/3 cup flour
• 1 cup sugar
• 3 egg yolks
• 1 cup water, divided
• 2/3 cup milk
• 2 tablespoons butter or margarine
• 1 teaspoon vanilla
• baked pie crust or purchased crumb crust, 9-inch