To prepare the buns, dissolve yeast in warm water in a large bowl; let stand for 5 minutes. Lightly spoon flour into dry measuring cups, and level with a knife. Add 2 cups flour, pumpkin, and next 5 ingredients (pumpkin through nutmeg); beat with a mixer at medium speed until smooth. Turn dough out onto a floured surface. Knead until smooth and elastic (about 10 minutes); add enough of the remaining 3/4 cup flour, 1 tablespoon at a time, to prevent dough from sticking to hands (dough will feel tacky).
Place the dough in a large bowl coated with cooking spray, turning to coat top. Cover and let rise in a warm place (85°), free from drafts, for 45 minutes or until doubled in size. (Press two fingers into the dough. If an indentation remains, the dough has risen enough.)
Combine 3 tablespoons granulated sugar, brown sugar, 2 tablespoons flour, and cinnamon in a small bowl. Cut in butter with a pastry blender or 2 knives until mixture resembles coarse meal.
Punch dough down; cover and let rest for 5 minutes. Roll the dough into a 12 x 10-inch rectangle on a floured surface. Sprinkle with brown sugar mixture. Roll up the rectangle tightly, starting with a long edge, pressing firmly to eliminate air pockets; pinch seam and ends to seal. Cut roll into 12 (1-inch) slices. Place slices in a 9-inch square baking pan coated with cooking spray. Cover and let rise 25 minutes or until doubled in size.
Preheat oven to 375°.
Bake the rolls at 375° for 20 minutes or until golden brown. Cool for 15 minutes in pan on a wire rack.
To prepare the glaze, combine the powdered sugar, 1 tablespoon water, and vanilla extract in a small bowl, stirring with a whisk until smooth. Drizzle glaze over buns. Serve warm.