Preheat oven to 350 degrees F. Line a 13x9" baking pan with foil, extending the sides of the foil over the edges of the pan. Spray the foil generously with cooking spray and set aside.
In a large bowl, combine the cake mix, egg and oil with a spoon until a soft dough forms. Press the dough evenly into the prepared pan and bake for approx. 12 minutes. Remove from the oven but keep the oven on.
Line up the Pop Tarts gently onto the crust. There should be four on top and four on bottom, but if needed, break up Pop Tarts to fit. Top first with the miniature marshmallows, then the chopped cookies and chocolate chips. In a small bowl, quickly whisk together the Biscoff spread and sweetened condensed milk until combined; pour the mixture evenly on top of the bars.
Return to the oven and bake for approx. 18-20 minutes or until the top is golden brown and bubbly. Center will appear just about set. Cool completely, preferably overnight.
Once bars are cool, cut into squares. Prepare the chocolate CandiQuik according to package directions or until melted and smooth. Pour the chocolate into a small resealable baggy and snip off a corner of the baggy. Drizzle the bars with the CandiQuik and immediately top with the Mallow Bits for garnish. Serve!