Line a 9x13-inch baking pan with foil and spray with cooking spray. Set aside.

In a large pot on the stovetop, add butter and melt over medium-high heat, stirring occasionally. Once the butter is melted add the marshmallows to the pot, reserving approximately 1/2 cup of marshmallows in the bag. Stir butter and marshmallows constantly until all the marshmallows have melted. Once the marshmallows have completely melted and incorporated into the butter, remove the pot from stove and stir in the salt, vanilla extract and chocolate chips. Stir until combined and the chocolate chips have mostly melted.

Gently stir in the cereal into the marshmallow mixture one cup at a time, adding the reserved 1/2 cup of marshmallows at the end. If adding add-ins, gently fold them in. Stir until the ingredients are incorporated. Pour mixture out into prepared pan and gently press mixture into pan. Top with colorful sprinkles (if desired), pressing gently onto the tops of the treats.

Let the treats cool in the pan and set before slicing into desired sized squares.