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

Remove from heat and stir in 1 cup of creamy peanut butter and 1 teaspoon of vanilla extract (if using). Mix until creamy.

In a large mixing bowl, add 6 cups of Rice Krispies cereal. Pour the melted mixture over the cereal and fold together using a spatula.

Prepare a muffin tin with liners or grease lightly. Spoon the mixture into liners, filling about ¾ full and press to compact.

Melt 1 cup of chocolate chips in the microwave in 20-second intervals, stirring until smooth. Spoon chocolate over each cup.

Let the cups cool at room temperature for about 20 minutes or refrigerate to firms up before serving.