Spray a 9 "x 9" (or 8 "x 8" for thicker bars) casserole dish with nonstick cooking spray.
Combine PBfit Pumpkin Spice with mix and stir until creamy. Set aside.
In a 4-quart saucepan, melt butter on low heat. Do this slowly; you don't want burned or brown butter here.
Once the butter has melted, keep heat on low and add mini marshmallows.
Once marshmallows melt, turn off the heat and stir in PBfit Pumpkin Spice and vanilla extract.
Once the mixture is combined, remove the pan from the stove.
Add rice crispy cereal to a bowl rubbed with butter or lightly sprayed with pan spray. Pour the marshmallow mixture into cereal and stir until the cereal is evenly coated with the mixture.
Pour the mixture into a casserole dish (it will look very loose). Spray your hand with nonstick cooking spray and pat down the mixture until firmly compacted.