Coat a 9x13" pan with butter.

In a large saucepan, melt butter over low heat. Add in marshmallows, stir and cook until completely melted and smooth. Remove from heat and stir in vanilla and salt.

Measure out rice krispies in a large bowl. Pour marshmallow mixture over cereal and gently stir to coat evenly.

Press mixture into prepared pan using either butter wax paper of the wrapper from your stick of butter.

Cool and cut into large squares