Place the marshmallows and butter in a microwave safe bowl and microwave for 1 to 1 1/2 minutes until fully melted, stirring every 30 seconds. Alternatively, you can melt the butter and marshmallows in a saucepan over medium-low heat by stirring for 3-4 minutes.
Add the rice krispies and pumpkin spice to the bowl, and stir to combine, or carefully work the mixture with your hands (it will be hot!), then add in the white chocolate chips.
Line a 9×13 inch pan with parchment paper, and press the mixture into the bottom of the pan in an even layer. Refrigerate for 30 minutes, then slice and eat!