Line a 9x13-inch baking pan with wax paper or parchment paper. Lightly grease the paper with butter or non-stick spray to prevent sticking.
In a large saucepan over low heat, melt 3 tablespoons of unsalted butter. Add the marshmallows and stir continuously until completely melted and smooth, about 5 minutes. Be careful not to overheat or burn.
Remove the marshmallow mixture from heat. Immediately add 6 cups of crispy rice cereal and stir gently but thoroughly until evenly coated.
Transfer the mixture into the prepared baking pan. Press firmly and evenly into the pan using a buttered spatula or wax paper.
Place 1 cup semisweet chocolate chips in a microwave-safe bowl. Microwave in 20-second intervals, stirring after each, until smooth and fully melted. Alternatively, melt in a double boiler.
Pour the melted chocolate over the pressed Krispie treats. Spread evenly with a spatula to cover every corner.
Allow to cool at room temperature until chocolate hardens, about 1 hour, or refrigerate for 20 to 30 minutes for faster setting. Once set, lift treats out using the parchment paper edges.
Cut into squares or rectangles with a sharp knife. Serve immediately or store for later enjoyment.