Make the bars: Preheat oven to 350F. Line an 8-in square baking pan with tin foil and lightly grease. Set aside.
In a large bowl, whisk pumpkin, sugar, oil, eggs, buttermilk, and vanilla until smooth. In a separate medium bowl, combine flour, pumpkin pie spice, baking powder, baking soda, and salt. Gently fold flour mixture into liquid until just combined. Evenly spread batter into prepared pan and smooth top with a spatula.
Bake bars at 350F 30-35 minutes or until a toothpick inserted in middle comes out clean. Cool bars completely in pan on a wire cooling rack.
Make the glaze: In a small bowl, whisk sugar, vanilla, cinnamon, and just enough milk to achieve a drizzling consistency.
Once bars are cooled, pour glaze over top and gently spread over bars. Allow icing to set 10 minutes, then slice bars into squares and serve.
Store bars in an airtight container at room temperature up to 3 days.