In a medium saucepan, melt 1/4 cup of unsalted butter and 3 cups of mini marshmallows over low heat, stirring constantly. This will take about 5-7 minutes.

Stir in 1 cup of creamy peanut butter (and 1 teaspoon of vanilla extract if desired) into the melted marshmallow mixture. Mix until fully combined and creamy; this should take around 2-3 minutes.

In a large bowl, add 6 cups of Rice Krispies cereal. Pour the melted peanut butter and marshmallow mixture over the cereal and gently fold it in.

Line a muffin tin with paper liners. Spoon the Rice Krispie mixture into each cup, pressing down gently to form a flat surface.

In a microwave-safe bowl, melt 1 cup of chocolate chips in 30-second intervals until smooth, about 1-2 minutes. Spoon the chocolate over each cup.

Allow the cups to cool at room temperature for about 20 minutes to set. For quicker results, refrigerate for 10 minutes before serving.