In a bowl, combine the sugar and flour. Add eggs; mix well. Stir in the pumpkin, milk, vanilla and, if desired, cinnamon; mix until well blended. Pour into a greased 9-in. pie plate. Place pie plate in a 15x10x1-in. baking pan; add 1/2 in. of hot water to pan. Bake at 350° until a knife inserted in the centre comes out clean, 50-55 minutes. Cool on a wire rack.