Preheat oven to 350. Prepare a 12-cup fluted tube pan with cooking spray and flour; set aside. In a mixing bowl, combine cake mix, pumpkin, eggs, 1/3 cup evaporated milk, oil, and half cup chocolate chips. Mix until well blended. Pour into prepared pan. Bake for 45 minutes. Cool in pan for 25 minutes. Then, invert onto wire rack to cool completely. Meanwhile, in a saucepan, heat remaining evaporated milk and cinnamon. Heat just to a boil then remove from heat. Add remaining chocolate chips, stirring until melted and smooth. Drizzle over cooled cake.