In a medium saucepan over low heat, combine unsalted butter and mini marshmallows. Stir until smooth, about 5 minutes.

Stir in creamy peanut butter and optional vanilla extract until well blended.

In a large bowl, pour in Rice Krispies cereal and fold in the peanut butter mixture until coated.

Scoop the mixture into muffin tin lined with paper liners and press down to shape the cups.

Melt chocolate chips in a microwave-safe bowl, then spoon over each Rice Krispie cup to coat.

Chill in the refrigerator for 20 minutes until chocolate sets, then serve.