Grease a 9 x 13" pan with butter or a nonstick cooking spray.
In a large microwave-safe bowl, combine your marshmallows and butter. Heat for about 1 to 2 minutes or until the marshmallows get puffy and then stir until well combined.
Add the Rice Krispies cereal and stir until the cereal is evenly coated with the marshmallow mixture.
Transfer the prepared rice Krispies treats into a greased pan and press it down on until firmly packed and leveled. Allow to set and cool for about an hour.
Using a jersey cookie cutter, cut the Rice Krispies into shapes and place on a flat work surface lined with a piece of parchment paper.
Fill one decorating bag with vanilla frosting and cut a small tip off the end of the bag. Outline the jerseys with the vanilla frosting.
Roll out the blue fondant in between two pieces of parchment paper and use number cookie cutters to cut out the jersey numbers.
Arrange the fondant numbers on the front center of the jerseys and press gently to attach.
Use the blue frosting to decorate the jerseys such as adding v-neck and stripes on the sleeves and on the bottom
Notes
Nutritional facts calculated only for the Rice Krispies treat and do not include the frostings and fondant.