Melt butter in large saucepan.
Melt 12 oz of marshmallows (roughly 3/4 of a bag) in the saucepan stirring constantly to avoid burning.
Add cereal and vanilla and stir until cereal is coated with marshmallow mixture.
Allow mixture to cool for about 3 minutes. When the mixture has cooled slightly, stir in remaining marshmallows. The marshmallows will melt a little, but stay somewhat intact.
Press mixture into a 9x13 pan lined with parchment paper. Allow treats to set for about 15 minutes.