Preheat your oven to 200ºc/180ºfan, and line two-three trays with parchment paper.
Add the unsalted butter, light brown soft sugar and granulated sugar to a bowl and beat until creamy.
Add in the egg and vanilla extract and beat again.
Add in the plain flour, baking powder, bicarbonate of soda and sea salt and beat until a cookie dough is formed.
Add in the chocolate chips and beat until they are distributed well.
Scoop the cookies using a 5cm scoop onto lined trays.
Bake in the oven for 11-13 minutes, or until golden.
Remove the trays from the oven and leave the cookies to cool fully on the trays.
Marshmallow Frosting
Add the egg whites to a large bowl and start to whisk to stiff peaks
Add the 100g of caster sugar, 1 tsp at a time, whilst whisking.
Once all the sugar is in, whisk in the vanilla and leave the meringue to whisk on a slow speed just to keep it moving and stop it from sitting.
Whilst this is whisking, add the other 100g of caster sugar to a pan with the water. Heat this on a low heat to dissolve the sugar.
Once the sugar is dissolved, turn up the heat and boil the mixture to 113ºc.
Once heated, turn the whisk up high again and trickle the syrup into the meringue to make the marshmallow. Once all the syrup is in, whisk for a further 5 minutes.
Assembly
Pipe the chocolate spread onto half of the cookies, and then pipe the marshmallow on top.
Sandwich the cookies together and enjoy!
Notes
You can freeze the raw cookie dough easily, and bake from frozen if you don't want to bake the entire batch - add 1-2 minutes to the baking time.
Once baked, these will last for 3-4+ days, in the fridge.