Christmas Wreath Cookies - Once Upon a Chef

Ingredients

• 4 cups cornflakes cereal, divided
• 1 stick (½ cup) unsalted butter
• 30 large marshmallows, from 1 (12-oz) bag
• ¼ teaspoons salt
• 1½ teaspoons green food coloring
• 1 teaspoon vanilla extract
• Red decorating icing with a plain piping tip, for berries (see note)
• Confectioners' sugar, for snow (optional)