Preheat oven to 350 degrees F. Liberally grease a 13x9" baking pan with cooking spray; set aside.

In a large bowl, whisk together the pumpkin puree, evaporated milk, pumpkin pie spice, sugar, eggs and vanilla with a whisk until smooth. Pour into the prepared pan. Top with the dry yellow cake mix, sprinkling evenly over and entirely covering the top of the pumpkin pie layer. Drizzle the yellow cake mix layer with the melted butter. Top with scattered handfuls of the mini chopped peanut butter cups, miniature Reese's Pieces, and peanut butter chips.

Bake uncovered for approximately 50-55 minutes or until the center is mostly set - it may wiggle slightly but you do not want to overbake it. Cool it completely, then refrigerate for at least 2 hours to set before cutting into slices. Just before serving, garnish with whipped cream and additional Reese's Pieces candies, if desired.