Ruth's Rice Krispies Squares

1/4 c. butter
5 c. mini-marshmallows
1/2 tsp. vanilla
5 c. Rice Krispies

*Melt butter in sauce pan. Add marshmallows, and cook over low heat, stirring constantly, until marshmallows are melted. Remove from heat: stir in vanilla.
*Add rice Krispies and stir until well-coated with marshmallow mixture.
*Press warm marshmallow treats mixture lightly into greased 13x9" pan. Cut into squares when cool.