Spray an 8" or 9" square cake pan with non-stick spray.
Remove ice cream from the freezer to thaw slightly.
Melt the butter in a pan on low heat. (I find that cast iron pots work best.)
Add marshmallows and melt on low heat. Then, add in vanilla.
Gradually stir in the Rice Krispies. Turn off the heat.
Scoop half of the cereal mix into the cake pan and press into shape. Cover the pot securely with a lid to keep the remaining cereal mix warm.
Put the cake pan in the freezer for 5 minutes to harden the cereal mix.
Once cooled, spread a layer of ice cream on top of the cereal mix to desired thickness. Put back in the freezer for 5-10 minutes.
Scoop remainder of the Rice Krispie Treat mix on top of the ice cream. Press into shape.
Pop the entire thing in the freezer for several hours or overnight before cutting and serving.