Grease your 13x9 baking dish with butter or cooking spray. Set it aside so it's ready when you need it.
Melt the 6 tablespoons unsalted butter and the entire 16-ounce bag of mini marshmallows together in a large saucepan over low heat. Stir constantly with a spatula or wooden spoon until the marshmallows are completely melted and the mixture is smooth and glossy. Low heat is key here — high heat can scorch the marshmallows and make the mixture grainy.
Add the 1½ tablespoons of cherry gelatin powder to the warm marshmallow mixture. Stir it in thoroughly until the powder is fully dissolved and the color is even throughout. You should see a uniform pink with no white streaks. If the gelatin isn't dissolving fully, the mixture might not be hot enough — give it another minute on low heat while stirring.
Fold in the 6 cups of Rice Krispies cereal. Use a gentle folding motion — don't stir too aggressively or you'll crush the cereal into tiny pieces. You want the marshmallow mixture to coat each piece evenly without breaking them down.
Press the mixture into your greased 13x9 baking dish. Use a buttered spatula or buttered hands to press it down evenly. If you want THICKER treats, use a 9-inch square pan instead — the same amount of mixture will fill it deeper. I lightly butter my hands and press gently — firm enough to hold together, but not so hard that the bars become dense.
Frost the treats with the melted dark chocolate. Pour the 10 ounces of melted Ghirardelli dark chocolate wafers over the top and spread it evenly with a spatula. If you're using sprinkles, add them now while the chocolate is still wet.
Allow the treats to set at room temperature for about one hour. The chocolate should firm up completely — it'll go from glossy to matte and feel firm to the touch. If your kitchen is warm, you can pop the pan in the fridge for 20-30 minutes instead.
Cut into squares and enjoy. I like to trim the edges first for clean-looking bars, but that's totally optional. Store any leftovers in a covered container at room temperature.