Line a 9- x 12-inch baking dish with parchment paper, and spray with cooking spray. Melt the butter in a large pot over medium heat. Add the marshmallows, and stir for 5 minutes, until they are completely melted.
Remove from heat, and stir in the vanilla. Add the Rice Krispies, and quickly stir to blend everything together.
Spread the mixture into the dish. Place a piece of parchment paper on top, and press down firmly to pack the mixture into the baking dish. Let it sit at room temperature for 30 minutes.
Cut the Rice Krispies treats into 15 pieces, and stick a thin sandwich stick or cake pot stick into one side. Melt the chocolate chips in a double boiler. Working on a parchment-lined board or baking pan, spread the chocolate on the bottom half of the treat, sprinkle it with sprinkles, and lay it on the parchment to harden. Let cool for 15 minutes, then serve.