Line bottom and sides of a 9x13 inch baking pan with foil, leaving an overhang on the sides.
In a medium bowl, whisk the first 4 ingredients together. Set aside.
With an electric mixer, cream butter and sugar on medium high speed until smooth, beat in egg and vanilla until combined.
Beat in pumpkin puree (don't worry if it looks curdley). Reduce speed to low and mix dry ingredients until just combined. Fold in butterscotch chips.
Spread batter evenly in prepared pan. Bake until edges begin to pull away from side of pan and a toothpick inserted in center comes out with just a few moist crumbs, attached, 35-40 minutes. Cool completely in pan.
Lift bars from pans (using the foil to help). Peel off foil, and use a serrated knife to cut into 24 squares.