Using a ladle, temper the eggyolk mixture by adding a small quantity of milk to the bowl and then whisking together quickly (don't allow the yolks to cook or you will get a grainy final product).
Continue adding milk in this fashion until you have a creamy liquid.
Add this mixture to the saucepan with the remaining milk and blend together well.
Using a wooden spoon, cook the sauce over a low heat, stirring constantly and making sure that the bottom is continually scraped.
Cook for about 5 minutes or until the sauce thickens (it will leave a clean line on the wooden spoon when you draw your finger across it).
Pour into a bowl and place bowl over an ice bath to cool.
Assembly[]
Cut marshmallow with round cookie cutter one size smaller than the graham cracker.
Place marshmallow on plate and torch with kitchen propane or butane torch (if not available, broil briefly under the broiler).
Place marshmallow on top of graham cracker and put on plate with malt sauce.
Place cocoa mousse in/on warm marshmallow and put onto the graham cracker.
Top with second graham cracker and serve immediately.