Lightly grease a 13x9 pan with butter or line with parchment paper. Set aside.
In a large saucepan over medium-low heat, melt the butter completely, stirring occasionally.
Add 10 oz (about 5 1/2 cups) of marshmallows. Stir frequently on low heat until just melted and well combined with the butter. Do not turn up the heat—this keeps them soft and chewy.
Stir in vanilla extract. Then add the Rice Krispies cereal and the remaining marshmallows (about 2 1/2 cups). Gently fold until combined and the extra marshmallows just start to soften.
If using sprinkles, stir them in briefly. Immediately transfer the mixture to the prepared pan. Use a silicone spatula to gently pat it into an even layer—do not press down hard or the treats will become dense.
Let cool completely at room temperature (about 30–60 minutes) before cutting into squares. Store in an airtight container at room temperature for up to 5 days.
Notes
Nutrition per serving: Calories: 99, Total Carbs: 15g, Net Carbs: 15g, Fats: 4g, Protein: 0g.