1/4 cup margarine or butter
5 cups miniature marshmallows
or 40 regular (250g pkg.)
1/2 tsp. vanilla extract (optional) 
6 cups Kellogg's Rice Krispies cereal
 

Recipe from Kellogg's
 
 
 
 

In a large saucepan over low heat, melt margarine.
Add marshmallows; stir until melted and well blended.
Remove from heat.

Stir in vanilla.
Add cereal, stirring until coated.

Using lightly buttered spatula, press into buttered 13x9" pan.
Cool.
Cut into squares.