In a large bowl, stir together oats, flour, baking soda, cinnamon, and pumpkin pie spice.
In another large bowl, whisk together butter, sugar, brown sugar, egg yolk, vanilla, and pumpkin puree.
Add the wet ingredients to the dry ingredients, and stir until just combined. Do not overmix - it leads to 'tough' cookies.
Fold in 1 1/2 cups butterscotch chips.
Drop by heaping tablespoonful (golf ball-sized) onto parchment paper or silicone mat-lined baking sheets.
Flatten slightly - cookies won't spread a whole lot.
With the remaining 1/2 cup butterscotch chips, dot 3-4 chips on the top of each cookie.
Bake at 350 degrees for 13-15 minutes, or until edges are golden, and the centers appear almost 'set'.
Remove from heat, and cool on cookies sheets for about 5 minutes before removing to wire racks to cool completely.