Line 9x13 pan with wax paper, and lightly spray with cooking spray. Make sure the wax paper hangs over the edge for easy removal from the pan once complete.
Set aside approximately 2 cups of mini marshmallows for later. This should be about half of one 16oz bag.
In a large pot, melt the butter over medium heat until lightly golden and foaming. Reduce to low heat and add mini marshmallows and vanilla extract. Stir over medium low heat until marshmallows are fully melted and mixture is smooth.
Remove pot from heat and add puffed rice cereal. Fold mixture gently until evenly combined allowing it to cool slightly. Add reserved marshmallows and sprinkles, and fold until combined. The reserved marshmallows should get gooey around the edges, but not fully melt.
Transfer mixture into prepared pan and gently press into even layer. Let cool for 1-2 hours before cutting.