For the marshmallow creme
Pop your egg whites and cream of tartar into a mixing bowl and whisk until they form soft peaks.
Add the sugar, syrup, water and salt into a saucepan and heat. Bring this to the boil until it reaches 240 degrees F/ 120 degrees C.
Once the syrup has reached the right temperature, slowly begin to mix it into the egg whites. Do this very slowly initially so that the eggs don’t begin to cook.
As you add more of the syrup, turn the whisk up higher. Beat the mixture until it is stiff and glossy. This should take roughly 8 mins. Towards the end of the beating time add in the vanilla paste.
It is now ready to use straight away, or can be stored in the fridge for 2 weeks.
For the cake
Bake the Chocolate Fudge Sponge according to the recipe, substituting the normal circular cake pans for rectangle pans.
Spread a thin layer of marshmallow crème over the top of the base layer and place the second layer on top.
Leave the marshmallow crème to set for 10mins or so before icing the top of the cake.
Spoon the marshmallow crème into a piping bag with a large round nozzle and pipe clouds of the crème on top of the cake.
Crush the chocolate digestives in a sandwich bag with a rolling pin before sprinkling on top of the cake.