Preheat your oven to 350°F (180°C) and fill a cupcake tin with 12 paper liners. Set aside.
In a medium sized bowl, combine the milk, apple cider vinegar and olive oil. Set aside to curdle for about 5 minutes. Some lumps may form which is normal.
In another bowl, whisk together the flour, sugar, baking powder, baking soda and salt. Pour in the milk mixture and add in the vanilla.
When there are no more large clumps of flour and the batter is relatively smooth (a few small lumps are fine), spoon the batter into the prepared baking pan until each cup is ⅔ full. Slide into the oven and bake for 15-20 minutes or until a skewer inserted at the center of a cupcake comes out clean. Cool in the pan for 5 minutes then transfer to a wire rack to cool completely.
Meanwhile, make the buttercream. In a stand mixer fitted with the whisk attachment, beat the the butter and sugar together until pale and fluffy, around 3-5 minutes. Add in the vanilla and salt and beat until you arrive at a silky frosting.
Place the frosting in a piping bag and decorate the cupcakes as you wish (see this blog post for more). Eat immediately or store at room temperature for up to 2 days*.