PREP PAN: Lightly grease a 9 x 13-inch baking dish with butter or nonstick spray. Set aside.
MELT BASE: In a large pot over low heat, add 6 tablespoons unsalted butter. Cook until fully melted and bubbling gently, but not browning.
MELT MARSHMALLOWS: Add 6 cups mini marshmallows and the optional ½ teaspoon kosher salt. Stir constantly until the marshmallows are mostly melted and the mixture looks smooth and glossy. You should smell a light, sweet vanilla aroma. Remove from heat as soon as melted.
ADD CEREAL: Immediately add 12 cups Lucky Charms cereal. Stir gently but quickly until all the cereal is evenly coated and sticky.
FOLD IN MARSHMALLOWS: Add the remaining 2 cups mini marshmallows and stir just until mixed in. You should still see whole marshmallows for extra gooey texture.
PRESS INTO PAN: Transfer the mixture to the prepared baking dish. Using a buttered silicone spatula or wax paper, gently press the mixture into an even layer. Do not press firmly or the treats will turn hard.
DECORATE: Melt the green candy melts or almond bark according to package directions. Drizzle over the top of the treats. Sprinkle with Lucky Charms marshmallows if using.
SET AND SLICE: Let the treats cool completely at room temperature, about 30 minutes, until set. Cut into 24 squares and serve.
Notes
Make-Ahead: These treats can be made up to 2 days ahead for parties or holidays.Storage: Store in an airtight container at room temperature for up to 3 days. Do not refrigerate or they will harden.Ingredient Note: The salt is optional, but it balances the sweetness and makes the flavors pop.Candy Melt Tip: If using almond bark, microwave 1 to 2 squares in 30-second intervals, stirring each time, until melted.Add green food coloring a few drops at a time until the desired shade is reached.