Heat oven to 180C/160C fan/gas 4 and fill a 12 cupcake tray with cases. Using an electric whisk beat the butter and sugar together until pale and fluffy then whisk in the eggs one at a time, scraping down the sides of the bowl after each addition. Add the vanilla, flour and a pinch of salt, whisk until just combined then spoon the mixture into the cupcake cases.
Bake for 15 mins until golden brown and a skewer inserted into the middle of each cake comes out clean. Leave to cool completely on a wire rack.
For the buttercream, whisk the butter until super soft then add the icing sugar, vanilla extract and a pinch of salt. Whisk together until smooth (start off slowly to avoid an icing sugar cloud) then beat in the milk. If wanting to colour, stir in the food colouring now. Spoon or pipe onto the cooled cupcakes.