Special equipment: Wooden ice pop sticks
Lightly butter 2 (12-cup) muffin pans.
Melt the butter in a large saucepan, over medium heat. Add the marshmallows and stir constantly until melted. Remove from the heat; stir in the peanut butter and vanilla. Add the cereal and 2 bottles of the sprinkles, stirring until combined. Using greased hands, press the mixture into the prepared pans, mounding mixture into a cupcake shape. Insert a wooden ice pop stick and let cool for 1 hour. Remove the rice cakes from pans.
In a medium bowl, combine the candy coating and vegetable shortening. Microwave on high, in 30-second intervals, stirring between each, until the chocolate is melted and smooth, about 2 minutes total time.
Dip the tops of the rice cakes in the melted chocolate. Immediately sprinkle with remaining sprinkles and serve.