Preheat the oven to 350F degrees. Line an 8x8 inch square baking pan with aluminium foil and grease with non-stick cooking spray or butter. Set aside.
In a medium sized bowl stir together the melted butter, brown sugar, vanilla & egg yolk using a large wooden spoon or rubber spatula. Be careful that the butter isn't too hot or you'll scramble the egg. Stir in the flour and graham cracker crumbs.
Pour/spoon the batter into your prepared pan to form an even layer that covers the bottom. Bake for about 12 minutes then remove from the oven.
Turn the oven to broil.
With the tray out of the oven - carefully pour the chocolate chips on top of the warm graham cracker layer. Wait a few minutes for the chocolate chips to begin to melt, then carefully spread them smooth using a rubber spatula. Then sprinkle the marshmallows on top in an even layer.
With the oven on broil, set the pan back in the oven to toast the marshmallows. This should take 2-5 minutes. Carefully watch your bars the entire time and remove from the oven when the tops of the marshmallows are golden brown.
Allow to cool at room temperature for at least 1 hour before cutting into bars. Or to speed up the process - allow to cool at room temperature for 10 minutes, then place in the fridge to continue cooling. The bars will still be very gooey and melty when cutting into them. If you'd like to make it easier, allow the bars to cool fully before cutting into bars.