Heavily grease a 9x13 baking pan.

Melt marshmallows, butter, and vanilla in a saucepan on medium until completely smooth.

Pour melted mixture over cereal and stir until coated.

Dump cereal mixture into greased pan (to make this easier, grease a piece of parchment or wax paper and use it to push the cereal into the pan).

Let sit until hard, then cut!