Add the bodies of the marshmallow chicks to a microwave-safe bowl. Microwave for 30 seconds to 1 minute until melted. Stir in the strawberry jam and cornstarch until thoroughly combined.
Pour the marshmallow mixture into a piping bag or zip-top bag. Snip off the tip and pipe the marshmallow mixture onto each whoopie pie bottom. Top with the whoopie pie tops. Take the reserved marshmallow chick heads and stick on top of the whoopeeps.
If the marshmallow chick head does not stick onto the whoopie pie top, heat for a second or so in a nonstick pan until sticky.
Cook's Note: For yellow marshmallow chicks, use lemon curd; for purple marshmallow chicks, use blackberry preserves.