In a medium saucepan over low heat, add the unsalted butter and mini marshmallows. Stir constantly for about 5–7 minutes until the mixture is completely melted and smooth.

Stir in the creamy peanut butter along with vanilla extract if using. Mix well for another 2 minutes until everything is fully combined and creamy.

In a large mixing bowl, pour in your Rice Krispies cereal and carefully fold the warm peanut butter mixture into the cereal using a spatula.

Prepare a muffin tin by lining it with paper liners. Spoon the Rice Krispie mixture into each liner, pressing down gently to form compact cups.

Melt the chocolate chips in a microwave-safe bowl in 30-second intervals, stirring in between until smooth, about 1–1.5 minutes total.

Drizzle the melted chocolate over each cup, spreading it evenly.

Allow the cups to cool at room temperature for 20 minutes or refrigerate for quicker setting, about 10 minutes.