In 1 1/2-quart microwavable bowl, microwave 1/4 cup butter, the milk, corn syrup and 1/8 teaspoon cinnamon on High 45 seconds or until mixture can be stirred smooth. Stir in chocolate chips. Microwave an additional 30 seconds; carefully stir until chocolate melts and mixture is smooth (stirring too quickly can cause air bubbles in glaze). Stir in 1 cup powdered sugar and the vanilla until smooth. Stir in additional powdered sugar, 1 tablespoon at a time, until consistency of thick syrup. Dip top of each doughnut into glaze; let excess drip off. Place doughnuts on cooling rack, allowing excess glaze to drip onto cookie sheet. Let stand until glaze is set, about 30 minutes.