Pour the sugar, cream, butter and corn syrup into a large pot. Set to medium heat and stir until everything is melted and combined.

Insert a candy thermometer and increase the heat to high. Once the mixture comes to a boil, stir occasionally until it reaches 240F.

Remove from the heat and let sit for 5 minutes, or until the temperature reduces to 230F. Add the vanilla extract and salt and mix well.

Mix vigorously with a wooden spoon until the fudge thickens, loses its shine and cools to 140F. Then mix for a few minutes longer.

Pour the fudge into a 9x9 inch baking pan lined with parchment paper, and smooth the surface. Let sit at room temperature overnight.

Slice the fudge into furniture and decorate with the remaining rainbow-coloured royal icing.