Preheat the oven to 350 degrees F and line a 9x13" pan with tin foil or parchment paper.
In a medium bowl, stir together flour and sugar. Cut butter into cubes and add to flour, mixing with a pastry cutter, mixer, or your hands until crumbly -- if it holds together when you squeeze it, it's ready.
Press into prepared pan and bake for 10-12 minutes, until dry looking but not browned.
Sprinkle peanuts, cashews and marshmallows over the crust after it has baked.
In a small saucepan over medium heat, stir together caramels and cream until melted and smooth.
Pour over nuts and marshmallows on crust, drizzling in a thin line to get as much coverage as possible.
Bake for 6-8 minutes, until caramel is just bubbling at the edges.