Line a bar pan with parchment paper. This recipe will fill half the bar pan, so plan to use only half the space or make a double batch!
Cook the butter in a large dutch oven over medium heat, stirring regularly, until it becomes golden brown and begins to smell nutty. Add the the canned pumpkin, marshmallows, pumpkin pie spice, and vanilla. Stir until the marshmallows are melted. Remove from heat.
Stir in the cereal and gently fold into marshmallow mixture until fully coated.
Transfer to the prepared pan and press in evenly with a piece of parchment paper. Let cool for at least 20 minutes until set at room temperature.
Once set, mix the icing. Cut the treats into squares and then drizzle the icing on top.