1. Place the dried Irish moss in a bowl and cover with cold water. Soak for 15 minutes (hydration: the moss will swell and soften). Drain and pick through the moss, removing any debris or shell fragments.
2. Combine the drained moss and 4 cups (960 ml) whole milk in the top of a double boiler. Set over simmering water and cook for 30 minutes, stirring occasionally. Do not extend the cooking time: the milk will appear only slightly thicker than when it started, but the carrageenan in the moss will set firmly once chilled. Over-cooking produces an excessively stiff result.
3. Remove from heat. Add 1/4 teaspoon salt. Strain the mixture through a fine-mesh sieve into a large measuring cup or spouted bowl, pressing lightly on the moss to extract liquid. Stir in 1 1/2 teaspoons vanilla extract, then strain a second time through the sieve.
4. Dip six individual moulds (approximately 4 oz / 120 ml each, roughly 2.5 inches / 6 cm in diameter) briefly in cold water — this prevents sticking. Pour the warm mixture into the prepared moulds, filling them to the top. Refrigerate until fully set, at least 2 hours or up to overnight. The blanc-mange is properly set when it holds a clean shape and feels firm but lightly springy when pressed through the mould.
5. To unmould, run a thin knife around the edge of each mould, invert onto a glass serving dish, and lift the mould away. Arrange thin banana slices around each mould on the dish and place one slice on top of each. Serve with sugar and cream on the side.
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