Line a 9x13-inch cake pan with foil or parchment paper. Set aside.
Add the butter and marshmallows to a very large microwave-safe bowl, and heat in 1 minute intervals, stirring after each one, until melted and smooth.
Stir in the vanilla and milk. Add the Rice Krispies and mix until completely coated.
Spoon the mixture into the prepared pan and spread out. Press firmly into a packed, even layer.
Allow to set at room temperature for at least 30 minutes before cutting into squares.
Video
Notes
This recipe can easily be halved to fit a 8x8 or 9x9-inch square pan.Leftovers can be stored in an airtight container at room temperature for up to 3 days. They also freeze well, wrapped individually, for up to 2 months. Thaw overnight at room temperature.