Preheat the oven to 160°C / 140°C fan / Gas mark 3 and grease the Dalek mould really well (spray grease is best for this).
Mix the water and colouring in a small bowl. Add the cocoa and buttermilk and mix until smooth. Put to one side. In another bowl, mix together the flour, baking soda and salt. Put to one side.
In a large bowl, beat the butter using an electric mixer until light and creamy (should take around 3 minutes). Add the sugar and vanilla extract, and beat for another 5 minutes. Add the eggs one at a time, beating until fully combined before adding another….