Add the vinegar to the milk and set aside to slightly curdle (about 5 mins).
Preheat the oven to 180°C (160°C fan).
Grease and line two 20cm sandwich tins with a circle of baking paper on the bottom,
Put the rest of the ingredients in a bowl, add the milk mixture, then mix until you get a smooth batter. you can do this by hand, but an electric whisk makes this much easier.
Divide the mixture between the two tins and bake in the centre of the over for 25-30 mins. don't open the door to check until at least 25 mins. Once done they should spring back when lightly touched on top and a skewer inserted in the middle should come out clean.
Leave the cakes in the tins for a minute to cool, then turn out onto a wire cooling rack to cool completely.
Meanwhile make the buttercream icing by mixing all the ingredients together in a bowl.
Once the cakes are completely cool, spread some of the icing on the top of one layer to completely cover it. Sandwich the other cake on top, then use the rest of the icing to cover the top and sides. Dust with cocoa powder or decorate with fruit or vegan chocolate pieces.