Preheat the oven to 180C/350F. Prepare an 8 inch springform pan by greasing it with butter, then cut out a circle to fit the pan out of parchment paper and line the bottom of the pan with it.
In a medium bowl sift together flour and cocoa powder.
In the bowl of a stand mixer cream butter and sugar until pale and fluffy. Mix egg and vanilla in a small bowl, then gradually add the mixture to the butter and sugar with the motor of the stand mixer still running. If the mixture separates, add a tablespoon from the flour and cocoa mix.
Mix the red food colouring with 2 tbsp of buttermilk until completely dissolved, then add it to the rest of the buttermilk and mix well.
Starting and ending with the flour mixture, alternate adding the flour and cocoa and buttermilk to the butter, sugar and egg mix. Stop the mixer once in a while to scrape the bottom with a spatula, continue mixing until your cake batter looks like a light, evenly blended frosting.
Mix the vinegar and baking soda together in a small bowl, it should froth up, then add it to the cake and mix well.
Pour the batter into the prepared springform cake pan and bake for 25 minutes or until a toothpick inserted in a cake comes out clean. Cool in the pan for 10 minutes, then remove from the pan and cool on a wire rack completely.
Cut off the domed top of the cake with a sharp knife if the cake didn't come out perfectly flat and reserve it for making crumbs for decoration.