Add brown sugar and softened butter to a medium mixing bowl and beat with the paddle attachment until combined and fluffy, about 1 minute.
Add pumpkin puree, egg and vanilla to the sugar mixture and beat for another minute until thoroughly combined.
In a small bowl, whisk together flour, baking soda, baking powder, salt and pumpkin pie spice until evenly distributed.
With the mixer on low speed, add the flour and mix until just combined.
Scrape the side and bottom of bowl to ensure everything is well mixed.
Allow the dough to rest in the refrigerator for at least 1 hour or overnight.
Take the dough out of the fridge and divide into 13 portions about 1 1/2 oz each. For precise measurement, use a kitchen scale.
Grease your hands with melted butter and roll the dough into balls. Set them on a parchment line baking sheet about 2” apart. Refrigerate while the oven is preheating.
Preheat the oven to 375°F.
Mix granulated sugar and pumpkin pie spice together for topping and set aside.
Grease the bottom of a measuring cup with melted butter and flatten the dough balls. It may be a little sticky still, slide the measuring cup from side to side to remove it from the cookie dough instead of lifting up.
Sprinkle the sugar mixture over each cookie generously.
Bake for 13 minutes.
Allow them to cool on the baking sheet for 8 minutes.
Press a BUTTERFINGER® skull into the middle of the slightly cooled cookies.
When all cookies are adorned with a BUTTERFINGER® skull, place the cookie tray in the freezer immediately to stop the candy from melting.
Flattening the dough ball will help the cookies spread and bake more evenly. However, this is optional. You can bake the dough balls as is without flattening them, they will just dome a little more.