Step 1: Melt the butter over low heat in a large pot, stirring regularly to prevent the butter from burning.
Step 2: Add marshmallows and vanilla extract to the pot. Stir the mixture until the marshmallows are completely melted, stirring frequently.
Step 3: Remove the pot from the heat and fold in the cereal.
Step 4: Transfer the cereal mixture to the prepared baking pan. Gently spread the mixture to fit the pan and press it down lightly
Step 5: Pour the melted chocolate over the Rice Krispie treats and spread the chocolate evenly. Transfer the pan to the fridge for 20-30 minutes, or set for 1 hour at room temperature.