Preheat oven to 350°F. Line an 8x8 pan with foil or parchment paper and grease it. Begin by prepping the pumpkin puree (see note).
In a large bowl whisk melted butter, brown sugar, and granulated sugar until fully combined, about one full minute. Whisk in egg, vanilla, and pumpkin puree.
In a separate bowl, combine flour, spices, baking powder, and salt. Add the flour mixture to the wet ingredients and stir with a wooden spoon or rubber spatula until combined. Stir in the butterscotch chips and white chocolate chips (I reserve a few to press into the top of the batter, as it makes for a prettier appearance).
Pour the batter into the prepared pan and spread into an even layer. Press some additional "chips" on top, if desired. Bake for about 23-28 minutes, until the top is dry and a toothpick inserted into the center comes out without raw batter on it, moist crumbs are OK. Place on a rack to cool completely.
Notes
PUMPKIN PUREE:In order to get the proper consistency, and to prevent the bars from being too wet, it is important to remove some of the moisture from the pumpkin puree.Place a double layer of paper towels on a plate. Spread the pumpkin puree onto the towels. Place a double layer of paper towels on top of the pumpkin and lightly press down allowing the paper towels to soak up excess moisture. Repeat this step one more time with fresh, dry paper towels. Allow the pumpkin to sit on the towels until you need it.MIX-INS:Feel free to swap the butterscotch chips or white chocolate chips for regular chocolate chips and/or nuts.