In a large pot, melt the butter over medium-low heat.
Add one bag plus 1 cup (about 7 cups total) of mini marshmallows to the melted butter. Stir until completely melted and smooth. Stir in the vanilla extract and a pinch of salt.
Remove the pot from heat and quickly stir in the rice crispy cereal until evenly coated. Gently fold in the remaining mini marshmallows.
Press the mixture evenly into the prepared pan. Avoid compacting it too much to keep the treats light and airy. If desired, top with sprinkles.
Allow the treats to cool completely before cutting into squares and serving.
Notes
Store in an airtight container at room temperature up to 3 days.