Place the butter in a bowl, whisk in the sugar and salt. Add the eggs and continue to beat until the mixture becomes lighter in colour. Stir in the milk. Mix the flour and baking powder, stir into the mixture and transfer to the prepared tin.
For approx. 20 mins. in the centre of an oven preheated to 180°C. Remove from the oven, allow to cool a little for about 10 mins., remove from the tin, leave to cool completely on a rack.
Using the whisk on a hand mixer, beat the butter and icing sugar for approx. 2 mins. until light and fluffy. Stir in the cream cheese and beat until smooth. Transfer the frosting to a piping bag with a serrated nozzle (approx. 10 mm in diameter) and pipe onto the cupcakes.