Preheat the oven to gas mark 4 / 180°C / 160°C if fan-forced. Line and grease 3 x 20cm springform pans with baking paper and butter.
To make your own buttermilk, add 4 tbsp of white vinegar to 300 ml full cream milk. Stir and allow to sit for 15 minutes until thick and curdled.
In a bowl, combine the flour, baking powder, bicarbonate of soda, cocoa and salt. Fork together.
In a stand mixer with the whisk attachment (or mixing bowl with a handheld mixer), cream together the caster sugar and butter until light and fluffy. Beat in the vanilla followed by the eggs, one at a time, mixing well after each addition.
Exchange the whisk for the paddle attachment. Slowly beat in half of the dry ingredients followed by the food coloring and 2 tbsp vinegar. The mixture should be bright red. Add the rest of the dry ingredients and mix until just combined before beating in the buttermilk. Do not overbeat the mixture as this will affect the lightness of the cake.
Divide the mixture equally into the prepared pans. Bake for 25-30 minutes (or 30-35 minutes if you have only 2 tins) on the middle shelf until a testing pin, stuck into the middle of the cakes, comes out clean. It may be wise to start checking after 25 minutes. Remove the cakes from the oven and allow to cool for 10 minutes in the pan. Thereafter, remove the cakes from the pans and allow to cool completely at room temperature, on a wire rack.
If you baked the cakes in only 2 cake tins, once the cakes are cooled use a sharp bread knife to evenly halve the cakes, giving you 4 layers. Cut without hesitation to ensure an even cut. If you used 3 cake tins, proceed to making the frosting.