In a large mixing bowl, gently crush the cornflakes. You want them to be slightly broken but not completely crushed.
Melt the unsalted butter over low heat in a saucepan. Once melted, add the marshmallows and stir until completely melted. Take it off the heat and incorporate the vanilla extract by stirring.
Pour the marshmallow mixture over the crushed cornflakes. Gently fold and stir until the cornflakes are evenly coated with the marshmallow mixture.
Line a baking sheet with parchment paper. Using a spoon or your hands, shape the mixture into individual clusters and place them on the parchment paper. Allow them to cool and set for about 20-30 minutes.
Once the clusters have set, they are ready to be served.