Mix the flour, sugar, baking powder, salt and lemon zest in a bowl. Whisk together the eggs, milk, butter and lemon juice, stir into the flour. Spoon the batter into the prepared muffin tin.
Approx. 30 mins. in the centre of an oven preheated to 180°C. Take the cupcakes out of the oven, allow to cool slightly, remove from the tin and leave to cool on a rack.
Combine the butter and icing sugar, stir in the cream cheese, lemon zest and juice. Transfer the mixture to a piping bag with a serrated nozzle (approx. 10 mm in diameter) and pipe onto the cupcakes. Decorate with a little lemon zest as desired.