Heat the oven to 180C/160C fan/gas 4. Line the holes of a 12-hole cupcake tin with paper cases. Stir the milk and vinegar in a jug and leave to thicken slightly for a few mins.
Beat the butter and sugar with an electric whisk until well combined. Whisk in the vanilla, then add the milk a splash at a time, alternating with spoonfuls of the flour. Fold in any remaining flour, the baking powder and a pinch of salt until you get a creamy batter. Don’t worry if it looks a little curdled at this stage.
Divide between the cupcake cases, filling them two-thirds full, and bake for 20 mins until golden and risen. Leave to cool on a wire rack.
To make the buttercream, beat the butter, icing sugar and vanilla with an electric whisk until pale and creamy. Divide between bowls and colour with different food colourings until you get desired strength. Spoon or pipe onto the cooled cupcakes.