Chocolate Chess Pie

1 - 1 1/4 - cups sugar (your preference) 3 - tablespoons unsweetened cocoa powder 2 - eggs 1 - (5 oz.) can evaporated milk (2/3 cup) 1/4 - cup melted butter, softened (1/2 stick) 1 - teaspoon vanilla extract 1 - refrigerated pie crust