Line a sheet pan with parchment paper or a silicone baking mat to prevent sticking and allow the cookies to set properly.

Place the marshmallows and butter in a large microwave-safe bowl. Microwave in 30-second increments, stirring in between, until the mixture is completely melted and smooth.

Stir in the green gel or liquid food coloring and vanilla extract until the marshmallow mixture is uniformly colored and smooth.

Add the cornflakes to the bowl and gently fold them into the marshmallow mixture, ensuring the cereal is evenly coated without crushing the flakes.

Lightly grease a 1/4 cup measuring cup with cooking spray. Pack the cereal mixture firmly into the cup, then turn it out onto the lined sheet pan. Use your fingers (damp or coated with cooking spray) to create a hole in the center to form a wreath shape.

Place red hot candies or red candy-coated chocolates around the wreath to resemble festive berries.

Allow the wreaths to cool completely at room temperature until firm, then serve or store in an airtight container.