Grease and line a 9 inch square tin with baking parchment. 

Start by beating the butter and sugar until well combined then beat in the vanilla. Next, beat in the whole egg and the egg yolk (keep the white aside for now) and then fold in the flour. Dollop into your prepared cake tin and chill for thirty minutes. 

When chilled, preheat the oven to 180˚C and set a shelf no higher than half way up in your oven - you don't want the brownie to burn. 

Melt the butter and chocolate together in a saucepan over low to medium heat then whisk in the sugars. Take off the heat. 

Beat in the 3 large eggs and the reserved egg white, followed by the vanilla. Fold in the flour. Pour on top of the chilled cookie dough layer then bake for 35-40 minutes until the brownie layer is dry on top but a skewer has one or two damp crumbs on it when inserted into the centre. It will rise very well. 

Whilst still hot, cover the surface of the brownies with marshmallows and leave in the tin until absolutely stone cold. They will start to melt and form a nice even layer by the time it's cold. 

When cold and the marshmallow layer is set, take it out of the tin, remove the baking parchment and using a blow torch, lightly toast your marshmallows. This should take seconds only. If you don't have a blowtorch, do this under a hot grill (broiler) - set the brownies now further away than 2 inches and stand and watch. The marshmallows will colour very quickly. 

When cold, the marshmallow will be firm enough to slice. Slice up your brownies using a very sharp knife that you have carefully oiled, wiping as needed. They slice very well. Dig in!