Preheat oven to 350°.
To prepare the cake, lightly coat a 10-inch tube pan with cooking spray; dust with 1 tablespoon flour. Spread pumpkin over 2 layers of paper towels; cover with 2 additional layers of paper towels. Let stand about 10 minutes. Scrape drained pumpkin into a bowl.
Place 3/4 cup granulated sugar, brown sugar, and 1/2 cup butter in a large bowl; beat with a mixer at medium speed 3 minutes or until well blended. Add eggs, 1 at a time, beating well after each addition. Beat in pumpkin and vanilla. Lightly spoon 3 cups flour into dry measuring cups, and level with a knife. Combine flour and next 4 ingredients (through salt) in a bowl, stirring well with a whisk. Add flour mixture and 3/4 cup buttermilk alternately to sugar mixture, beginning and ending with flour mixture.
Spoon batter into prepared pan. Bake at 350° for 55 minutes or until a wooden pick inserted in center comes out clean. Cool in pan 10 minutes on a wire rack. Remove from pan, and cool completely on wire rack.
To prepare glaze, combine 1/3 cup buttermilk and remaining ingredients in a small saucepan over medium heat; bring to a boil. Cook 1 minute or until thick, stirring constantly; remove from heat. Drizzle cake with glaze.