In a medium saucepan, over low heat, combine 4 tablespoons of unsalted butter with 2 cups of mini marshmallows. Stir continuously until smooth, about 3-5 minutes.

Once melted, remove from heat and stir in 1 cup of creamy peanut butter and optional vanilla extract until well combined.

In a large mixing bowl, pour in 4 cups of Rice Krispies cereal. Fold in the peanut butter mixture until evenly coated.

Prepare a muffin tin lined with paper liners. Spoon the mixture into each cup, pressing gently to form a solid base.

In a microwave-safe bowl, melt 1 cup of chocolate chips in 30-second intervals. Spoon the melted chocolate over each Rice Krispie cup.

Let the cups cool at room temperature for about 30 minutes, or refrigerate for 20 minutes until firm. Peel away paper liners and enjoy!