Prepare a 13x9 pan by lining with foil and generously greasing with butter (you may also use un-buttered wax paper or parchment paper, if desired). Set aside.
Cut your butter into 8 pieces and place in a large saucepan over medium-low heat.
Stir frequently until butter is melted. Increase heat to medium-high, stirring constantly.
The butter will begin to bubble and pop, continue to stir until butter just begins to turn golden brown and immediately remove from heat.
Add marshmallows and stir well, until completely melted (if the marshmallows are struggling to melt, you can return the saucepan to a burner on low heat).
Once marshmallows are melted, stir in vanilla extract and pumpkin spice.
Stir in cereal. Immediately pour into prepared 9x13 pan and spread evenly.
Scatter evenly with sprinkles, if using.
Allow to cool before topping with chocolate.
Prepare your white chocolate by placing in a small microwave safe bowl and heating for 30 seconds. Stir well and return to microwave, heating in 15 second increments and stirring well after each until completely melted.
Use a spoon to drizzle chocolate over the top of your cooled pumpkin spice rice krispie treats. I like to add on a few more sprinkles at this point, but this is optional.
Allow chocolate to harden before cutting and serving.