Line a sheet pan with parchment paper or a silicone baking mat.

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

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

Add the cornflakes to the bowl and gently fold them into the marshmallow mixture.

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. 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.