Step 1: Line an 8x8 inch baking pan with parchment paper, leaving an overhang on the sides for easy removal. Set aside.
Step 2: In a microwave-safe bowl, combine the milk chocolate chips and unsalted butter. Microwave in 30-second intervals, stirring well after each, until the chocolate is smooth and fully melted. Alternatively, melt over a double boiler.
Step 3: Stir in the vanilla extract until fully combined with the melted chocolate mixture.
Step 4: Add the puffed rice cereal to the chocolate mixture. Gently fold with a spatula until the cereal is evenly coated. Be careful not to crush the cereal.
Step 5: Pour the mixture into the prepared baking pan. Using the back of the spatula or your hands (lightly greased if needed), gently press the mixture evenly into the pan.
Step 6: Chill the bars in the refrigerator for at least 30-45 minutes, or until completely firm.
Step 7: Once firm, use the parchment paper overhang to lift the slab from the pan. Place on a cutting board and cut into desired bar sizes using a sharp, warm knife.