In a medium saucepan, melt ¼ cup of unsalted butter and 4 cups of mini marshmallows over low heat, stirring continuously for about 5 minutes until melted and smooth.

Once the base is slightly cooled, stir in 1 cup of creamy peanut butter and 1 teaspoon of vanilla extract until fully incorporated.

In a large mixing bowl, combine 6 cups of Rice Krispies cereal with the peanut butter mixture and gently fold the cereal in for about 2-3 minutes.

Line a muffin tin with paper liners and spoon the Rice Krispie mixture into each cup, pressing down gently.

Melt 1 cup of chocolate chips in a microwave-safe bowl, stirring every 30 seconds until smooth. Spoon a thin layer over each Rice Krispie cup.

Let the treats cool to room temperature or refrigerate for 20-30 minutes until firm.