Preheat the oven to 170ºC.
Grease and line your cake tin with greaseproof paper.
In the bowl of a stand mixer or large mixing bowl, cream together the butter and sugar until light and fluffy.
Add the eggs and mix well.
Add the cocoa powder, red food colouring and vanilla extract and mix well.
Add half the buttermilk, pouring slowly as the mixer is on.
Add half the flour and mix well.
Add the rest of the buttermilk and flour, and mix well.
Add the salt, bicarbonate of soda and vinegar and mix together well.
Pour the mixture into the prepared cake tin and bake for 30-35 minutes. Leave the cake to cool before cutting out the heart shapes and assembling.