Lightly grease a 13x9 pan with butter or line with parchment paper and set aside.
Place butter in the bottom of a large saucepan over medium-low heat. Stir occasionally until butter is completely melted.
6 Tablespoons European salted butter*, cut into 6 pieces
Add 10 oz (about 5½ heaping cups) 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 (5oz/about 2 ½ heaping cup). Stir until ingredients are combined and additional marshmallows are just beginning to melt.
1 teaspoon pure vanilla extract, 6 cups Rice Krispies cereal
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).
3 Tablespoons colored sprinkles
Allow Rice Krispie Treats to cool completely at room temperature before cutting and serving -- about 30-60 minutes.
Video
Notes
Butter
You can substitute with regular salted butter, but I really like the flavor and quality of European.
Storing
Store in an airtight container at room temperature for up to 5 days. I do not recommend refrigerating or freezing.