Add sugar, cornstarch and milk to a large saucepan over medium heat and mix until smooth. Stir constantly until it begins to thicken. Reduce heat and continue to cook for about 2 minutes.
Remove from heat and add about 1 tablespoon of milk mixture to eggs, whisking until smooth. Slowly add 2 more tablespoons and pour egg mixture back into saucepan. Return to medium heat and bring to a boil, stirring until mixture thickens, about 2 minutes.
Remove from heat and add butter and vanilla and transfer to a bowl. Place plastic wrap directly onto the custard and place in fridge to chill.
Roll out green fondant and use a large round cookie cutter to cut out green circles about 1/2 inch large than the cupcakes.
Roll out black fondant and use a paring knife to cut the black hair and eyebrows. Brush a thin layer of water on the green fondant to stick on hair and eyebrows. Lightly score the face of the monster to make a mouth and add candy eyes.
Fill piping bag with custard and pipe onto cupcakes. Gently cover cupcakes with monster faces, pressing the edges onto the sides of the cupcakes.