Preheat oven to 375°.
To prepare cookies, lightly spoon flour into dry measuring cups; level with a knife. Combine flour and next 6 ingredients (flour through salt); stir well.
Combine pumpkin, brown sugar, and butter in a large bowl; beat with a mixer at medium speed until light and fluffy. Add milk, vanilla, and egg whites; beat well. Gradually add flour mixture to pumpkin mixture; stir until combined.
Drop dough by heaping tablespoonfuls onto baking sheets coated with cooking spray. Bake at 375° 10 minutes or until almost firm. Cool on pans 1 minute on a wire rack; remove from pans. Cool completely on wire rack.
To prepare glaze, combine powdered sugar, milk, vanilla, and cinnamon; stir well with a whisk. Dip tops of cookies in glaze. Place cookies, glaze side up, on wire racks; allow to set.