Preheat your oven to 170ºC/150ºC fan and line/grease two 8" cake tins - leave to the side for now.
Beat together the unsalted butter and caster sugar in a stand mixer until smooth and fluffy like you would a normal cake, and then gradually beat in the egg so that it doesn't curdle (if it doesn't curdle, beat in a little flour to bring it back)
In a small bowl, mix together the cocoa powder, Sugarflair red extra food colouring, and vanilla extract to a thick dark paste (if it is struggling to become a paste, then add in a little milk to make it runnier and mix better - it needs to be smooth!) - it may take a couple of minutes but this will make it easier to add to the rest of the mixture if you do it this way - and the sponge will be more red!
Add this mixture to the unsalted butter and caster sugar mixture and beat until combined and evenly coloured.
Turn the speed down to slow, and pour in half of the buttermilk.
Add in half of the plain flour and beat again, and then the other half of the buttermilk & beat, and then the other half of the flour & beat (I realise this is faffy, but it works!).
Beat in the bicarbonate of soda and white wine vinegar.
Beat again for a couple of minutes until everything is smooth and incorporated well.
Separate into the two tins and bake in the oven for 30-35 minutes until the middle of the cake comes out clean when poked with a skewer!
Once the cakes are baked, leave to cool fully on a wire rack.
Cream Cheese Frosting
Make sure your unsalted butter is at room temperature. I leave mine out overnight when it's cold weather, but in the hotter months this can take as little as 30 minutes!
Beat your butter on it's own for a few minutes to loosen it.
Add in the icing sugar, and beat again - I beat this for about 5 minutes, to make it really smooth.
Make sure your cream cheese doesn't have any excess water - I find it best to add it to a bowl first just to make sure.
Add in the cream cheese, vanilla and beat. At first, it may look a little weird, but just keep on beating.
I end up beating it for a few minutes - it can go through a lumpy stage first, but eventually the lumps beat out and it's smooth and thick!
Once beaten - it should be lovely and thick.
Pipe/spread 1/2 of the frosting onto the top of one sponge.
Add the other sponge on top, then pipe/spread the other half of the frosting onto the top and decorate with sponge crumbs if you fancy!
Notes
I really really really recommend in investing in good quality Sugarflair Red Extra food colouring for a red velvet cake - I use Sugarflair paste colours which you can buy online, or in cake decorating shops etc - but they don't have a funny taste to them, you use far less to make them work, and they last so much longer!!
This cake will last in the fridge for up to 3 days, covered, or you can freeze it for up to 3 months!