Method

• 01
  Place a fine-mesh strainer over a stainless steel or glass bowl. In a medium saucepan over medium-low heat, melt the butter. Remove the saucepan from the heat and add the sugar, and lime juice; whisk to blend. Whisk in the egg yolks until the mixture is smooth.
• 02
  Place the saucepan back on the heat and cook while whisking constantly until the mixture reaches about 180 F to 190℉ on an instant-read thermometer, or until it coats the back of a spoon. To test with a spoon, dip a metal or wooden spoon into the custard.
• 03
  Take it out and draw your finger over the back of the spoon through the custard film to make a path. If the path remains clear and doesn't fill in, the custard is done. Do not let the mixture boil. It will thicken as it cools.
• 04
  Pour the hot custard mixture through the mesh strainer. Discard the solids left behind in the strainer. Stir the finely grated lime zest into the mixture, if using.
• 05
  Place a sheet of plastic wrap directly onto the surface of the custard to prevent a skin from forming. Refrigerate for 3 to 4 hours, or until thoroughly chilled. Heat the oven to 325℉. Arrange 12 graham cracker squares on a large rimmed baking sheet.
• 06
  If desired, char the marshmallows slightly on a skewer over a gas burner torch. Place a marshmallow on each graham cracker square. Bake for 5 to 7 minutes, or until they are puffed and very soft.
• 07
  When the marshmallow-topped graham crackers come out of the oven, tamp the soft marshmallows down slightly to cover the cracker. Let them cool slightly and then invert a chilled lime curd-topped graham cracker onto each one. Serve at once.