In a medium saucepan heat the butter until melted.

Add the marshmellows and vanilla extract and stir constantly until it's all melted.

Add the cereal and mix until all the cereal have been covered with marshmallows.

Pour this into a sprayed 9x12 pyrex and push it down and into the corners. Set aside.

Melt the chocolate bars in the microwave, carefully removing it to mix every 30 seconds.

Once the chocolate have melted (about 2 mins total), pour it over the rice krispies.

With a flat spatula spread the chocolate evenly over the entire surface.

Refrigerate for at least 1 hour before cutting into bars.