Follow the directions on the back of the box, but add:
1/2 cup crunchy peanut butter to the warm marshmallows.
If you want peanut butter and chocolate rice krispie treats add 1/2 of you favorite chocolate chips to the marshmallow mix while still warm.
Add the rice krispie treats to a lightly buttered dish of your choice. I usually use a disposable tin cake pan.
Let cool and then enjoy.