Mix the cornstarch (cornflour) with the cranberry juice in a small pan, stirring well to dissolve. Add the vanilla extract and corn syrup (or liquid glucose) and stir to combine.
Heat over low heat, stirring constantly, for 3–5 minutes until the mixture thickens enough to coat the back of a spoon. Do not boil. If the fake blood becomes too thick you can thin it with more water or juice – remember it will thicken as it cools.
Take the pan off the heat and stir in the red food coloring, a little at a time, until you have the desired color. You can add a drop or two of blue or brown food coloring for a darker color.
Transfer to a jar and keep in the fridge for up to three weeks. You may need to stir in some additional cranberry juice or water and heat gently to bring to liquid consistency after chilling.
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Notes
Add the food coloring off the heat and check the shade by dripping a little onto a white plate – it looks much darker in the pan than it does on a cake.The blood thickens as it cools, so take it off the heat while it is still slightly runnier than you want it.Makes about 1 cup (250ml). Nutrition is per tablespoon.