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Start by making the Marmite. Soak the sourdough in the water with the yeast and sugar for 12 hours, then remove all the bread, squeeze out the liquid and discard the solids. Ferment the liquid for two days at room temperature then place over a high heat and reduce the liquid to a thick Marmite consistency (which will take several hours). This should leave you with about 200ml of Marmite. Once reduced and thick, transfer to a container and set aside