Heat oven to 160C/140C fan/gas 3 and line 2 baking trays with baking parchment. Put the butter and vanilla into a large bowl, and beat until soft and creamy. Add the colouring and beat to combine. Sift the remaining biscuit ingredients into the bowl and mix together until it comes together to form a uniform dough.

Using your hands, roll the dough into small balls (I weigh these so each biscuit is the same size, 20g each), then place 10 onto each prepared baking tray, leaving a couple of inches between each biscuit. Using a fork dipped in flour, press onto each of the biscuits, leaving an indent and pressing the biscuits a little flatter.

Pop the trays into the fridge for about 15 mins or until the biscuits have firmed up slightly, then bake for 20-25 mins. Allow them to cool on the trays for 5 mins before moving to a wire rack to cool completely.