Grease 3 x 7" round sandwich tins.
Preheat the oven to 180C.
Cream your room temperature butter with your sugars until it’s pale, light and fluffy.
Mix in the eggs, one at a time, beating really well after each egg. Don’t worry if it starts to look a bit curdled, it’ll all come good.
Mix in your flour,cocoa, mixed spice and cinnamon. If you’ve been using a mixer, do this bit by hand. It helps to keep the sponge light.
Stir in the milk.
Spoon into a your tins, smooth out with the back of a spoon. Make a bit of a dip in the middle to account for rising. (I am a bit OCD and tend to weigh mine into the tins to make sure they are all the same size, there should be about 800g of batter in each tin.)
Bake for about 30 minutes or until a skewer comes out clean. You might need to adjust the time by 5 minutes either way depending on your oven.
Meanwhile make your icing. Beat your butter until it's pale, light and fluffy.
Beat in your icing sugar and vanilla. Beat in a teaspoon or two of boiling water to soften and lighten the texture of the icing.
When your cakes are completely cold, use a carving knife to level them. Just trim the top crust off. This is important, it's not a waste of cake (taste testing compulsory). If you don't do this, with a cake this size it will be wonky. Then trim each sponge in half so you end up with 6 layers.
Colour 200g of buttercream red and 200g green.
Sandwich each layer together with alternating coloured buttercream.
Use the remainder of the buttercream to cover the cake and pop some swirls on top.