Lightly grease a square 8 or 9-inch (20 or 23-cm) glass or metal baking dish with butter. Set aside.
Place ¼ cup of butter in a microwave-safe mixing bowl. Heat the butter until melted. Add the marshmallows to the melted butter. Stir the marshmallows to coat. Return the bowl to the microwave and heat the marshmallows on full power until they look puffy and melted about 1 to 2 minutes. Add the peanut butter and vanilla. Stir until smooth.
Add the Rice Krispies to the bowl. Stir until all of the cereal is coated and the marshmallow is evenly distributed. Lightly butter a piece of parchment paper and use it to gently press the cereal into the buttered dish. Set aside.
Make The Chocolate Glaze
Place the butter and corn syrup in a small microwave-safe bowl and heat until the butter is melted. Add the chocolate chips to the bowl. Return the bowl to the microwave, and heat for 20 to 30 seconds more. Add the vanilla and stir smooth.
Use a spoon to drizzle the chocolate randomly over the Rice Krispie treats until gone. Let the chocolate set for about 20 to 30 minutes before slicing and serving.
Notes
The amount of time it takes to melt the butter and marshmallows depends on your microwave's power.
If the marshmallows are still holding their shape after stirring, continue to cook and stir in 15-second increments until smooth.
Do not press the cereal mixture into the pan too tightly. You just want to smooth it out evenly, not flatten it. If you press too tightly it can make the squares hard.
Made in a square pan, the treats are approximately 1 ½ to 2 inches (4 to 5 cm) thick. If you like thinner squares, spread the cereal mixture into a buttered 9x13 inch (23x33 cm) pan.
Spread the chocolate glaze into an even layer if you prefer that look.
I haven't tried freezing peanut butter Rice Krispie bars because they don't last long enough to bother. I usually store mine in a large zipper-top bag on the counter at room temperature for a couple of days.