How to Make It
Prepare the Bars: Preheat oven to 450°F. Toss together squash and oil on a rimmed baking sheet lined with aluminum foil; spread in an even layer, and roast in preheated oven until golden brown and very soft, about 18 minutes. Let cool 10 minutes, transfer squash to a food processor, and process until mostly smooth, about 1 minute. Reserve 1/2 cup of puree; discard remaining puree or save for another use.
Lower oven temperature to 350ºF. Butter an 8-inch square baking dish, and line with parchment paper, allowing 1 inch of paper to hang over sides of dish. Butter the parchment paper.
Melt butter in a medium saucepan over medium-low; cook, stirring and scraping bottom of pan often with a rubber spatula, until butter begins to turn golden brown and fragrant with a delicious nutty aroma (you should see bits of browned milk solids), 2 to 3 minutes. Turn off heat.
Whisk in brown sugar until smooth. Set aside until sides of pan are cool enough to touch, but still warm, about 15 minutes. Whisk in reserved squash puree, egg, cashew butter, vanilla extract, and salt. Stir in flour until all ingredients are incorporated.
Spread mixture into prepared dish. Bake at 350°F until golden brown around edges and a wooden pick inserted in center comes out clean, 23 to 25 minutes. Cool completely on a wire rack.
Prepare the Frosting: Beat together cream cheese, cashew butter, and butter with an electric mixer at medium speed until smooth and fluffy. Beat in powdered sugar until smooth. Spread frosting over cooled bars. Sprinkle evenly with cashews and, if desired, pepitas. Cut into 8 bars.