Put the marmite, water and sugar in a saucepan, bring to the boil and leave to simmer for 5 minutes. This helps to mellow the marmite flavour.
Pour the hot liquid on the chocolate and stir it to melt the chocolate, you'll need to use a stick blender to emulsify the liquid as the chocolate and water will not initially want to mix together. Leave this to set for at least 24 hours in the fridge and give it a really good stir before you use it.
Next make the macaron shells, preheat the oven to 170 degrees and line four baking trays with baking parchment.
Add the water and sugar to a pan and stir over a medium heat until the sugar dissolves.
Bring the syrup to a boil and heat until it reaches 118 degrees.
Put 75g of the egg whites into a stand mixer and whisk them on medium speed until frothy.
Pour the syrup in a slow steady stream into the egg whites as they are being whipped on a high speed. Keep whipping until the meringue forms a stiff peak, about 5 mins.
Grind the ground almonds, cocoa powder and icing sugar together in a food processor and then sift them into a clean bowl.
Add the remaining egg whites to this mix and beat it, until it forms a paste.
Add a third of the meringue to the almond paste and beat it in thoroughly.
Fold through the remaining meringue, ensure the ingredients are mixed thoroughly and that the macaron mixture falls from the spoon or spatula.
Put the macaron mix into a piping bag and pipe out 72 macarons.
Tap the bottom of the tray to remove any bumps or air pockets, then put the macarons in the oven for 12mins per tray. Turn the trays halfway through cooking to ensure an even bake.
Let the cooked macarons cool completely before trying to remove them from the tray.
Match the macarons up into similar sized pairs and gently make a small indentation with your thumb on the flat side of the macaron, then fill with the marmite ganache.
Keep the macarons in an airtight container for at least an hour before eating, but if you can wait, they'll be much better the next day. They should last for 2-3 days in the fridge.