In the bowl of a stand mixer or in a large bowl with a hand mixer, cream butter and brown sugar on medium speed until light and fluffy, 3-4 minutes.
Add pumpkin puree, pumpkin pie spice, and salt and blend until combined. With mixer on low, slowly add flour to bowl until well combined. If dough is too sticky, add additional flour 1 Tbsp at a time.
Remove pan with cookie crust from freezer and press cookie dough into an even layer over cookie crust.
Spread marshmallow sauce over the top of the cookie dough, using as much as you want, and smooth evenly with a spatula (note that marshmallow sauce will drizzle off the bars once they're removed from the fridge, so the more you use, the "messier" they'll be). Add sprinkles over the top.
Place pan in fridge chill for at least an hour. Once chilled, remove bars from pan by lifting foil. Using a sharp knife, slice into 25 squares, or however many you want.