Place butter in the bottom of a large saucepan over medium-low heat. Stir occasionally until butter is completely melted.
Add 10 oz (one bag) of marshmallows and stir until just melted. Keep your heat below medium and stir frequently, this will take a bit of time for the marshmallows to melt but too-high heat can cause your rice krispie treats to come out too hard.
Stir in vanilla extract, and then add rice krispies cereal and remaining marshmallows. Stir until ingredients are combined and additional marshmallows are just beginning to melt.
Scatter sprinkles over the mixture and briefly stir to combine. Immediately transfer mixture to prepared 13x9 pan and use a silicone spatula or your hands to gently pat rice krispie treats down evenly into pan (don't over-handle them or this will also make them come out too hard).
Allow Rice Krispie Treats to cool completely at room temperature before cutting and serving -- about 30-60 minutes.
*You can substitute with regular salted butter, but I really like the flavor and quality of European